Four million voters will elect 8,377 representatives to local and regional assemblies among about 64,000 candidates, said Reuters and AFP.
The agencies carried political analysts' opinions according to which the elections are a sort of test for the centre-right Ivo Sanader government.
According to the German agency dpa, voters might punish Sanader for failing to keep the main promises from his election campaign. There have been no new jobs, living standards have not risen and Croatia has not started European Union entry negotiations, dpa said.
This agency estimated that the Social Democratic Party (SDP) will certainly win in the capital of Zagreb. Other agencies spoke of a fierce battle for Zagreb and the two next largest cities, Rijeka and Split.
Reuters and AFP said the latest public opinion polls indicated that SDP-led parties will win in 12 to 13 counties, while Sanader's Croatian Democratic Union (HDZ) looks set to win in eight or nine.
The Slovene agency STA said the results will be uncertain until the very end, citing polls according to which the HDZ will win 26.2 per cent and the SDP 26 per cent of the vote.
About 6,500 polling stations were opened at 7 am and will close at 7 pm, when electoral blackout ends. The release of any preliminary results or estimates is also banned until then.