By Kylie MacLellan and Adrian Croft
RIGA (Reuters) - British Prime Minister David Cameron on Friday began his campaign to persuade European leaders to make changes to the European Union before he holds a referendum to decide whether Britain should stay in or quit the bloc.
Cameron was due to hold talks with European Council President Donald Tusk and a handful of EU government heads on the sidelines of a European Union summit with six ex-Soviet republics in Latvia.
There were no formal meetings planned with EU power brokers Germany and France but Cameron will travel to Berlin and Paris next week for talks with Chancellor Angela Merkel and President Francois Hollande, British officials said.
Cameron said the negotiations would have "ups and downs".
"But one thing throughout all of this will be constant and that is my determination to deliver for the British people a reform of the EU so they get a proper choice in that referendum," he told reporters on arrival at the summit.
Cameron's Conservatives won an unexpected parliamentary majority in a May 7 election and he is now committed to a pledge to hold a referendum on EU membership by the end of 2017.
He wants to restrict EU citizens' access to Britain's welfare system, opt out of "ever closer" union inside the bloc and to cut EU "red tape", but countries may be reluctant to make more concessions to London, which already does not participate in the euro currency or the passport-free Schengen area.
Cameron held talks with Polish Prime Minister Ewa Kopacz and was due later to meet Hungary's Viktor Orban, who added to his reputation in Europe for controversy by sending voters a questionnaire asking if they thought EU mismanagement of immigration was linked to a rise in terrorism.
Cameron also planned to meet Swedish Prime Minister Stefan Lofven and Latvia's Laimdota Straujuma.
Cameron, who intends to set out his reform plans in more detail at an EU summit in June, has said he wants Britain to remain in a reformed EU, but has not ruled out campaigning for an exit if he fails to get the required changes.
A number of EU leaders said they would listen to Cameron but there were limits to what they could agree to.
Lithuanian President Dalia Grybauskaite said free movement of workers was a core value of the EU. She urged everyone to "be consensual and not just try to receive additional opt-outs" at other countries' expense.
European Parliament President Martin Schulz told reporters debates with Cameron were "always difficult".