Chris McCall – Fearful Indonesian Christians in Sumatra's main city prayed behind a police guard last night after a bomb exploded at morning service and two more were found at other churches.

Thirty-three people received medical treatment after the home-made explosive device went off at the GKPI Protestant church during a morning youth service. Many of the injured were teenagers. The blast reverberated across the city.

Of the other bombs found later, one at a church metres from the local police headquarters was detonated by a bomb squad. The other was found in a plastic bag at a third church, sources said.

Police refused to speculate on who might have been behind the clearly co-ordinated bombing attempts, but radical Muslim groups in Indonesia have made a series of thinly veiled threats against Christian targets in recent months. This follows widespread anger among Indonesia's majority Muslim population at massacres of Muslims by Christians in the eastern Maluku Islands.

Medan, a city of two million, has a mixed population of Christians and Muslims but an unusually high proportion of Christians, one of the highest of any major Indonesian city.

"We don't know who did it. It is still being investigated," a police spokesman said. Local police chief Brigadier-General Soetanto called for calm and urged the Christian community not to panic or be provoked into any violent reaction. He called an urgent meeting of community leaders and ordered police protection at churches for evening services, amid fears of new attacks.

Police pledged to search for any new devices and warned of the dangers of Medan degenerating into religious bloodshed like that seen in the Malukus.

Fighting between Christians and Muslims there has killed thousands since it began early last year. Clashes between Muslims and Christians, which broke out on May 22 in the town of Poso in central Sulawesi, have spread to surrounding districts, police said yesterday.