Croatia's prime minister has ordered an inquiry following arrests of several opposition activists who made plans via the social networking website Facebook.
"This is not about this or that government or party, but about freedom," Croatian PM Ivo Sanader said.
Police in Zagreb questioned a Facebook activist who had put up posters ahead of an anti-government protest planned for Friday, Croatian TV reported.
Last week a man who had set up an anti-Sanader forum was held in Dubrovnik.
In a statement on Wednesday, Mr Sanader said he had asked Interior Minister Tomislav Karamarko and Police Director Vladimir Faber "to submit a report today on the latest events and arrests in Zagreb and Dubrovnik and to take appropriate steps if police did not respect regulations".
He said "no-one should be detained or arrested in Croatia for expressing different views".