An Indonesian prison chief and one of his inmates, believed to be a government official sentenced for graft, have been charged over a bombing on Sulawesi island that killed 21 people, police said.

National police chief Da'i Bachtiar told reporters that evidence pointed overwhelmingly to the involvement of the two men in Saturday's attack on a busy market place in the Christian town of Tentena.

The pair have been named as Hasman, the head of the prison in the nearby Muslim-dominated town of Poso, and Abdul Kadir, said to be an official jailed for embezzling state funds intended for the victims of religious conflict.

"It's those two. It's already clear that it was them, the head of the prison and Abdul Kadir," Bachtiar said.

Bachtiar said the pair were detained because police had found chemicals on the body of Hasman and inside Kadir's car that were "identical" to those used in Saturday's bombings.

He said Kadir had been picked up by police "wandering outside the prison" during a routine check on Monday night.

The weekend attack was earlier blamed by police on Islamic militants with possible links to the Jemaah Islamiyah organisation, the alleged Southeast Asian arm of the Al-Qaeda network, hoping to revive religious tensions.

Central Sulawesi has been dogged by violence between Christians and Muslims after a 2001 peace deal ended almost a year of fighting in which more than 1,000 people died.

But local officials have said the bombing could be a politically motivated to justify a strong military presence or an attempt to divert attention away from a corruption scandal.