The explosive was hidden in a garbage bin about 200 yards (meters) from where Karamanlis was about to advocate his re-election in Sunday's national vote, and even closer to some of his supporters, said police spokesman Panayiotis Stathis.
A police statement said two parked motorcycles were damaged by the blast, on a street off a downtown Athens avenue that was full of Karamanlis' supporters. The glass entrance to an apartment building also was smashed.
"It caused minor damage. There are no reports of injuries," Stathis told The Associated Press.
The time bomb exploded shortly after 8 p.m. (1700 GMT), minutes before Karamanlis took to the stage for his address _ which went ahead as planned. No group immediately claimed responsibility for the blast, which followed an anonymous call to a national newspaper that provided a 10-minute warning.
As Greece prepares for Sunday's election, Karamanlis' conservative party is trailing the main opposition Socialists by at least 6 percentage points, according to opinion polls released Sept. 18 _ the last date one could be published under Greek law.
Attacks in Greece by far-left militants and anarchist groups have increased sharply since widespread riots in December, sparked by the fatal police shooting of a teenager in Athens.
On June 17, militants shot dead a policeman guarding a terror trial witness in central Athens. A little-known group calling itself Sect of Revolutionaries claimed responsibility for the attack.
Authorities had believed that years of far-left terrorist violence had come to an end after the arrest of several members of the country's deadliest group, November 17, following a botched bombing in 2002.
But in 2007, a group called Revolutionary Struggle fired a small anti-tank rocket at the U.S. Embassy in Athens, causing minor damage and no injuries. That group has also stepped up its activity since the December riots, severely wounding a riot policeman in a January shooting and planting a powerful bomb outside the Athens Stock Exchange on Sept. 2, causing extensive damage but no injuries.