Attackers fired a rocket at the US embassy in Athens on Friday but no one was hurt, police and the US embassy said.
"This was a rocket attack launched from a building across the street. It landed inside a toilet on the third floor of the embassy," a senior police official said.
"There are no injuries from the blast," a US embassy spokesman said.
The senior police official said Greece's deputy police chief and Athens police chief had gone into the building together with officers of the national security and anti-terrorist squads.
Dozens of police cars surrounded the embassy and police cordoned off all roads in the area, including a major boulevard in front of the mission.
The tightly guarded US mission is surrounded by a three-metre-high steel fence. Guards are posted at every entrance and at street corners around it.
Local residents called in to state television saying that they had felt the explosion, which shattered some windows.
Greece's biggest domestic security threat, the leftist November 17 guerrilla group, which had in the past killed US and other foreign diplomats in Greece, was dismantled in 2002.
In November 2006, Greek riot police fired tear gas to disperse demonstrators marching to the US embassy in Athens who chanted slogans including "Bush the butcher, out of Iraq" and "the USA is the real terrorist".
In February 1996, unidentified assailants fired a rocket at the US embassy compound in Athens, causing minor damage to three diplomatic vehicles and some surrounding buildings.