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.
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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".
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