The Greek government says there is no evidence that a bomb attack on a police station in Athens is linked to the Olympic Games, due to open in 100 days.
Cars were wrecked and windows in nearby buildings shattered
Speaking hours after three bombs went off in the central Kalithea district, officials expressed confidence the Olympics would be safe for all.
No-one has claimed the attack which caused damage but no injuries.
Police are increasingly convinced that left-wing domestic groups are to blame and not international terrorism.
The BBC's Richard Galpin in Athens says the government is very keen to show that this attack was not linked to the Olympics, but was instead a violent reaction to the trial of alleged members of hardline leftist group accused of a series of bombings and killings in Greece over the past three decades.
A newspaper in the city received an anonymous telephone warning about 10 minutes before the pre-dawn blasts.
The bombs were each reportedly made from three sticks of dynamite triggered by alarm clocks.
Police said they believed the timing of the explosions was staggered in the hope that the second and third bombs would injure police officers arriving at the scene after the first blast.
Government spokesman Theodoros Roussopoulos said there was no evidence so far "to indicate that today's incident is linked to preparation for the Olympics.
"This is an isolated incident which does not affect whatsoever the country's preparations for the safety of the Olympics," Prime Minister Costas Karamanlis told reporters.
"The Greek people's efforts and their close co-operation with the relevant authorities in the European Union, Nato and the United States guarantee the safety of the Athens Olympic Games," he said.
The incident has raised awareness of the huge task that faces the Greek security authorities.
The Athens Games will be the first summer Olympics since the 11 September 2001 attacks on the US.
Athens has allocated tens of thousands of police officers and army personnel to ensure security at the games.
A city-wide network of surveillance cameras will be in operation, and aerial patrols will take place.
A Greek delegation, led by Public Order Minister George Voulgarakis and the head of the Greek police, is currently in Washington for talks on the effort to safeguard the games.
Some US officials have expressed worry that construction delays at Olympic venues could undercut efforts for advance security testing and other measures.
The British Prime Minister, Tony Blair, said he was confident the Greek authorities were doing all they could to ensure the Olympic Games passed off safely.
However, Australian Foreign Minister Alexander Downer said
Australia's intelligence agencies would reassess the threat of a terrorist attack at the Olympics.
A spokeswoman for the International Olympic Committee, Giselle Davies, said no formal approach had been made to the organisation by any national Olympic committee.
"Obviously it's an incident which is not connected with an
Olympic venue," she said, adding: "We understand it's a relatively small incident."
History of attacks
The Greek authorities claimed they had crippled the most dangerous domestic "terrorist" threat after the convictions last year of 19 members of the November 17 group, blamed for 23 killings and dozens of other attacks since 1975.
But smaller groups have continued to carry out bombings and arson attacks in Athens and other cities.
In September, bombs damaged a judicial
complex in Athens and injured one police officer. The bombings were claimed by a group
calling itself Revolutionary Struggle.
According to the police, the group threatened more violence after a bank was bombed in March. Also, the language used in Wednesday's tip-off was the same as that used in March.
Last month, the US state department's annual report on
terrorism said the "low-level bombings against an array of
perceived establishment and so-called imperialist targets... underscore the lingering nature of left-wing terrorism in Greece".