Ambon – A bomb ripped through a bus station on Indonesia's Ambon island late Wednesday injuring six people, witnesses said, amid tight security for a pro-independence rebel anniversary.

One person was seriously wounded in the blast in Ambon city and all six were taken to nearby Al- Fatah hospital, witnesses said. An 11-year-old boy was among the injured.

"At the moment, six people are injured and they have been taken to hospital. One of them is a child," one witness told AFP.

Local police told ElShinta radio that the small explosion occurred at 8:30 pm and officers were at the scene questioning witnesses. There were no immediate claims of responsibility.

One of the injured in hospital said shortly before the blast she heard four men talking to each other about why the bomb have not yet gone off.

"There are four men standing near me and they talked to each other anxiously. I heard them saying why it had not yet (exploded)," Siti Hasnah Umarela told AFP in the hospital.

Thousands of police and soldiers fanned out across Ambon city and towns on the island earlier in the day amid concerns of unrest for the anniversary.

Indonesian army chief Djoko Santoso said last week 6,000 police and military personnel would guard several strategic locations, particularly in Ambon city, the capital of Malaku province.

Two years ago, a pro-independence parade in the city led to clashes and a week of violence that left more than 30 people dead and more than 100 injured.

April 25 marks the 57th anniversary of the claim by the outlawed rebels to an independent South Maluku Republic, a territory covering the eastern Maluku island chain.

The separatist movement was crushed shortly after its declaration in 1950 but the rebels regrouped following the fall of dictator Suharto in 1998.

Ambon has also been ravaged by clashes between Muslims and Christians, which erupted in January 1999. A peace pact in February 2002 mostly ended three years of strife that left more than 5,000 people dead and hundreds of thousands homeless.