A small grenade exploded after it was fired at a building near U.N. offices in a downtown Beirut square early Sunday, injuring four people, police said.

Police and army troops surrounded Riad Solh Square after the grenade was fired from a rifle and hit the building, which houses dance clubs. It could not immediately be determined who was behind the attack and whether it was politically motivated.

The explosion broke windows in the building and damaged two cars, witnesses said. The injured, all civilians, were taken to a hospital for treatment. Two were later released.

A heavily fortified building housing U.N. offices is located in the same square, as is a building containing international and Arab media offices. The prime minister's office is about 100 yards away and the Parliament building is on another square a few buildings behind Riad Solh.

The area is popular with Arab tourists from Persian Gulf countries, some of whom have started returning to Lebanon after the end of the summer war between Israel and Hezbollah militants.

Earlier this month, two bombs were thrown at two police stations causing minor damage but no injuries. Several days earlier, a clash between police and rioters left two boys dead.

A U.N. Security Council cease-fire ended the fighting between Israel and Hezbollah that killed hundreds of Lebanese and scores of Israelis and destroyed parts of southern Beirut.