A bus was hit by a massive explosion outside Jerusalem's central bus station today, medical sources told AFP, saying around 20 people had been wounded, some of them seriously.

The explosion occurred shortly after 3:00 p.m. (1300 GMT) and shook buildings hundreds of metres away, witnesses said.

An AFP correspondent at the scene saw people lying on the floor covered in blood, and many cars and buses with shattered windows.

Medical officials quoted by Israel's Channel 2 television said four of the injured were in serious condition, and another two were moderately hurt.

It was not immediately clear if the blast took place on the bus or near it, and officials could not confirm whether it was an attack.

Sirens echoed through the city as dozens of ambulances and fire engines raced to the scene.

An AFP correspondent at the scene said the attack hit the 174 bus which was headed for Maaleh Adumim, a sprawling Jewish settlement which lies just to the east of Jerusalem.