Visitors to the Eiffel Tower have been evacuated after an anonymous phone call announcing an attack. The call came from the Paris suburbs and threatened an attack at 9.30pm (0730 AEDT). The anti-terrorism unit is investigating. Bomb alerts targeting monuments and public places happen frequently in the French capital - and the tower was evacuated at least once last year and twice in 2011 - but all alerts are "treated very seriously," the police source said. Special police units are currently inspecting the monument. The tower was reopened around two and a half hours later after police searches failed to find any suspicious objects. Security has been stepped up across France amid concerns about threats to the country during its military campaign in its former colony Mali.