(Adds details, background)

ADDIS ABABA, Nov 4 (Reuters) - Gunfire erupted in the Ethiopian capital on Friday, witnesses said, in a fourth day of clashes betwen police and anti-government protesters that have killed at least 42 people.

A Reuters reporter said police shot in the air to disperse protesters gathered close to the African Union headquarters in Addis Ababa, and saw groups of youths being arrested.

There was no immediate word on casualties.

Clashes first broke out on Tuesday when riot police confronted stone-throwing demonstrators apparently heeding a call by opposition leaders for a renewed protests against a May 15 poll which they say was rigged.

Ethiopia's worst violence in months has fuelled fears about the stability of the Horn of Africa's dominant power, prompting the European Union and African Union to urge restraint.