The leader of the Socialist (Pasok) party, George Papandreou, said the government was "totally incompetent".
Correspondents say the fires are likely to dominate political debate ahead of a general election on 16 September.
The PM said the poll would proceed as planned, but his priority was the victims, not political point-scoring.
"We are continuing a determined struggle against the fires," Costas Karamanlis said.
The BBC's Malcolm Brabant, in the capital Athens, says Greece has the feel of a country on a war footing, and says it is awash with theories about who could have set fire to the land.
Rescuers have evacuated 27 villages in the Peloponnese as the fires threatened to engulf them, but there have been no fatalities since Monday, reports say.
On Monday, terrified residents in Frixa in western Peloponnese were airlifted to safety by helicopters after being cut off by fire and thick black smoke.
Isolated villages have been cut off in the region, sparking fears that the death toll could rise further.
In Athens, hundreds of people took to the streets in protest.
Some of the demonstrators said they believed the fires were deliberately started by property developers in order to clear the land for development.
However, the president of the Hellenic Property Federation, Stratos Paradias, told the BBC that the suspicions were unfounded.
"That's absolutely nonsense, this allegation, because if there is a company which wants to get its hands on some piece of land, to burn this land is the worst way to do it.
"Once a forest is burnt, according to our constitution, within three months the Forestry Service is obliged to pass a decree according to which this land is reforested."
Meanwhile, seven people have been charged with arson and a further 26 face lesser charges linked to the fires that have blighted Greece since July.
Anti-terrorist squads are involved in the investigation, and a 1m euro (£678,000) reward has been offered to help catch those responsible.
The government declared a state of emergency over the weekend as new fires, fanned by hot, gusting winds, continued to break out around Greece.
Last Mod: 28 Ağustos 2007, 13:32