Greek police say about 3,000 anarchists, left-wing group supporters and students are marching through central Athens, to protest President Barack Obama’s visit.

A second planned Athens protest by Greek Communist Party (KKE) supporters is due to start later Tuesday.

Police will block the marchers from the part of the city where Obama will be, as authorities have banned all demonstrations in a wide swathe of central Athens.

Greek authorities have deployed more than 5,000 police for Obama’s two-day visit. The riot squad is on high alert for violence, and an armed anarchist group has called for "attacks and clashes" to disrupt the visit.

There is a strong anti-American tradition among Greek leftists, who still resent US support for Greece’s 1967-74 military dictatorship. [AP]