By Peter Salisbury and William Maclean
SANAA/DUBAI (Reuters) - The Houthi rebels who stunned the Arab world with the sudden seizure of Yemen's capital will have to strive to cement their power in the face of well-armed rivals, a test of strength that could tip the unstable country deeper into turmoil.
A suicide bombing last Thursday that killed 47 in Sanaa is an example of the indiscriminate tactics Yemenis fear some armed groups are prepared to employ to check the ascent of the Shi'ite Muslim movement that swept into the city on Sept 21.
The attack was claimed by al Qaeda in the Arabian Peninsula (AQAP), apparently furious at the ability of Houthi fighters to take over Yemen's fragile state and dictate terms to its embattled President Abd-Rabbu Mansour Hadi.
The mood in Sanaa is apprehensive.
"This is just the beginning," said Mohammed Saleh, a taxi-driver who ferried passengers away from the scene of the blast.
"It is a war now between al Qaeda and the Houthis and those who will be hurt the most are ordinary people."
His prediction chimes with those of analysts who worry that the bombing - one of more than 10 attacks across the country since the Houthi takeover that has killed as many as 100 - is energizing AQAP's anti-government campaign in Yemen, which in turn is taking on increasingly sectarian overtones.
"The situation is extremely dangerous," said a Yemeni official who declined to be identified due to the sensitivity of the matter. "We don't want a civil war, but all the indications are pointing to this."
It is not only al Qaeda that is upset by one of the most unusual changes of power in the Middle East since the "Arab Spring" of 2011.
The Houthis' rise has scrambled the prospects of a range of political forces, including an old guard of tribal, religious and military powerbrokers, as well as southern secessionists who seek to break away from the northern region of the country.
The Houthis' sidelining of President Hadi's weak and fractured administration capped a spectacular rise to national importance for a once obscure group from Yemen's remote northern highlands.
DANGEROUS
All are struggling to make sense of the ascent of a group that is an ally of Iran, foe of Saudi Arabia and self-proclaimed champion of Zaydi Shi'ites who make up about a fifth of Yemenis.
Perhaps most startling of all, say diplomats, is the likelihood that the takeover was made possible by an alliance between the Houthis and Ali Abdullah Saleh, a former president who was once their sworn enemy.
If that is true, it is richly ironic: The movement sees itself as resurrecting Yemen's Arab Spring-inspired 2011 uprising against what protesters saw as the autocracy and corruption of Saleh's 33-year rule.
The fall of Sanaa alarmed Saudi Arabia, which fears Iran is opening a new front in its Middle East Cold War with Riyadh, and threw into confusion a U.N.-backed transition funded by Gulf donors meant to steer Yemen to pluralism after years of dictatorship.
In a move that lowered tensions a little, Hadi on Monday named the country's U.N. envoy, Khaled Bahah, as prime minister, part of an agreement that stipulates the return of state authority and withdrawal of Houthi fighters from Sanaa.
But the nomination also underscores Houthi influence: Bahah's name had been among three proposed by the Houthis after the movement publicly and brusquely dismissed Hadi's appointment of Ahmed Awad bin Mubarak as prime minister last week.
SENSE OF UNCERTAINTY
Will the Houthis now call on their fighters to leave the streets? Yemenis ask. The presence of the fighters, residents say, has led many Yemenis to leave the city. This has hurt business and deepened a sense of uncertainty.
"I want people to leave their guns at home, but now there are thousands of them (guns) around," said Mohammed Ali, a 20-year-old street trader selling bottles of perfume from a plastic bag, referring to Houthi fighters.
How the Houthis took a city of two million with relatively little bloodshed, and what they will do now with their new found clout, are questions closely debated in Sanaa.
The answers to both questions, say diplomats, lie in a mix of populist ideology, good organization and power politics.
The Houthis see themselves as sweeping away a clique of tribal, religious and military powerbrokers they say buried the 2011 uprising under layers of compromise, sleaze and subservience to foreign powers.
The Houthis emerged in the north in the 1990s demanding an end to the materialization of Zaydi Shi'ites and fought a brief border war with Saudi Arabia from 2009-10.
Some Yemenis suspect the Houthis dream of reviving the Zaydi Imamate, the 1,000-year-long rule of Yemen in which power was handed down through Shi'ite leaders claiming descent from the Prophet Mohammed. The imamate ended in a 1962 military coup.
A GAME OF ALLIANCES
The Houthis are seen as apostates by the Sunni hardliners of al Qaeda. Saudi Arabia designates them as terrorists.
But their anti-establishment message plays well among Sanaa's large Zaydi Shi'ite population, which gives the group a natural support base in the city. Other communities are drawn to their populist economics: The group is a stout defender of state subsidies for fuel, for example.
Ideology is only part of the story.
Analysts say two relationships lie behind the Houthis' rise to power: A tactical alliance with veteran strongman Ali Abdullah Saleh, who stepped down as president in 2012, and a more strategic tie-up with Shi'ite power Iran.
Saudi Arabia fears that the Houthis seek to replicate the strategy of Iran's ally Hezbollah in Lebanon, using popular support among Shi'ites combined with a muscular military presence to dominate politics and project Iranian might.
Hadi has accused Iran of interfering in Yemeni affairs by backing unidentified armed groups.
Iran denies meddling in Yemeni politics. But an Iranian official told Reuters his country had "always backed Houthis".
INFLUENCE IN THE ARMY
The Houthis acknowledge they have friendly ties with Iran and share its anti-Western ideology, but deny they get military training or weapons from the Islamic Republic.
The Yemeni official said that as Houthi forces advanced on Sanaa in the days leading up to the takeover, the Yemeni government asked Iran via Omani mediators to use its influence with the Houthis to persuade them to not to invade the city.
The answer, the official said, was not helpful. "The Iranian response sounded quite like the Houthi response," he said.
Ibrahim Sharqieh, an expert on Yemen at the Brookings Doha Centre, told Reuters the Houthis sought to emulate Hezbollah. "They want to decide who rules Yemen, but they don't want to rule themselves," he said.
But the relationship that enabled Houthis to enter Sanaa was a tacit alliance with former president Saleh, who still wields influence in the army.
Some diplomats say he used that clout to get pro-Saleh army units to stand aside and let the Houthis take over.
POLITICAL RIVALRIES
Saleh's motives for the tie-up, diplomats speculate, originate in Yemeni political rivalries: The takeover was a defeat for Ali Mohsen, a politically powerful army general who split violently with Saleh in 2011, and whose forces made a failed attempt to stop the Houthis taking Sanaa in September.
"What changed is their forging of an alliance with Saleh. They both wanted to take revenge against Ali Mohsen. Ali Mohsen was the common enemy," said Sharqieh.
Many so-called People's Committees, bodies formed by the Houthis to oversee the operations of the government, appear to be made up of Saleh loyalists rather than Houthi supporters.
At a People's Committee checkpoint bedecked with a Houthi banner in Sanaa's Bier Abu Shamla district, a stronghold of Saleh support, Ali Lutf al-Haiwara said he joined the People's Committees on Sept. 21, as the Houthis entered the capital.
Saleh, he said, was the "best president ever", and he supported Saleh's political party, the General People's Congress, and the Houthis because "they have the same aims".
For his part, Saleh denies collaborating with the Houthis. The Houthis themselves have not addressed the issue directly, but say they have no problem with the former president.
How long the marriage of convenience lasts is a matter of speculation in Sanaa.
Has Saleh, a canny political manipulator, met his match?
"The Houthis are the only powerful group in the country," said the Yemeni official. "They have a vision. They are well disciplined, they are well organised. All the other groups ... are in disarray."
(Additional reporting by Parisa Hafezi in Ankara, Sami Aboudi and Rania El Gamal in Dubai and Angus McDowall in Riyadh; Editing by Giles Elgood)