Facebook has made it easier for activists to organise rallies
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".
In the Zagreb case, an opposition Facebook group with nearly 60,000 members included volunteers who had downloaded posters over the internet, Croatian TV reported.
The man arrested in the Croatian capital was charged with disturbing the peace, under an old law from 1990 which applied to the then-Yugoslavia, the TV reported.
The activist in Dubrovnik had set up a Facebook group called "I bet I can find 5,000 people who dislike Sanader".
Police argued that his group had illegally shown a photo montage of Mr Sanader in a Nazi uniform.
On Monday Mr Sanader said he deplored any use of Nazi symbols for the purposes of political satire.
The EU is closely watching Croatia's reforms of its justice system as the country is next in line to join the 27-nation bloc.