A bomb exploded outside the office of the American jeans company, Levi Strauss, in the northern Halandri area of the capital.

The time bomb blew out windows and damaged cars parked near the building, but caused no injuries.

Reports said a group calling itself Anti-Capitalist Action issued a deliberately misleading warning message just moment before the blast.

President Clinton's visit is opposed by many Greeks

In the second attack, six shots were fired at the building of the Hellenic-American Union in Athens city centre. Witnesses saw two men leave the scene on a motorcycle. They caused minor damage but no injuries.

No one has admitted responsibility for the attack, but police believe a group called Red Line, which has attacked American targets in the past, was behind the shooting.

There have been three other minor incidents in the past week and recent US newspaper articles have accused the Greek authorities of not doing enough to clamp down on terrorism.

Correspondents say the attacks appeared to be linked to protests against President Clinton's visit to Greece on Saturday.

Earlier on Sunday, a Greek anarchist group said it had set off a home-made bomb at a car showroom in Athens. The group said the attack was its way of "welcoming" Mr Clinton.

Tight security

Greek police plan to deploy thousands of extra men for the Clinton visit. American security teams began arriving in Athens last week.

The president's trip is opposed by many Greeks, angry at the US Government's leading role in Nato's bombing campaign against Yugoslavia earlier this year.

Greece, itself a Nato member, provided logistical support for the campaign, but Greeks largely sympathised with fellow Christian Orthodox Serbs during the Kosovo crisis.