An explosion rocked the Indonesian city of Medan in North Sumatra province late on Monday evening, but there was no immediate report of casualties, police said.

"There was a blast in Patumbak, near the Amplas [bus] terminal but there was no report of casualties," Second Sergeant PM Simanjuntak of the Medan city police headquarters said.

Simanjuntak said that the blast took place in front of an empty land plot at around 9pm and added that an anti-bomb squad was on the site combing the area for evidence and clues. He declined to comment further but said that the blast caused no damage to nearby buildings.

Witnesses at the site said that the blast followed the throwing of a small package from a speeding vehicle. Some said it involved a car, others said it was two men on a motorcycle.

Medan City Police Chief Senior Commissioner Bagus Kurniawan told journalists at the site of the blast that the explosion was due to a home-made bomb.

"It is really a home made bomb of a low-explosive type," Kurniawan said according to the Detikcom online news service. "This is purely an act of terror. We cannot yet predict what the motives of the perpetrator is beside of spreading terror," he added.

Detikcom also quoted an unnamed police source at the site of the blast as saying that police have found remains of parts of a PVC pipe believed to have been some 25-centimeters-long and nine centimeters wide, two lamp bulbs, an alarm clock, wirings and nail-like metal objects.

Indonesia has seen a series of deadly bombing attacks since 2000 targeting churches and places popular with foreigners, such as bars and hotels.

A bomb exploded at a music concert in Peureulak, Aceh, on New Year's Eve, killing 11 people and injuring many others. The military have blamed the separatist Free Aceh Movement (GAM), which is active in Aceh province north of North Sumatra province.

The most serious bomb attacks killed 202 people, mostly foreign tourists, in the international resort island of Bali in October 2002, when two bombs exploded near two popular nightclubs.

The government has accused the local Muslim militant Jemaah Islamiyah network, which has been linked to al-Qaeda, for several of the bombing attacks.

Indonesia has been rocked by a series of explosions since 2000 targetting churches, and sites popular among foreigners in various cities.