Filipino Catholics and Chinese Buddhists offered prayers for eight Hong Kong tourists at the site where they died in a bus hijacking, and the Philippine government promised a credible investigation into what went wrong.

Justice Secretary Leila De Lima said on Tuesday she expected a report by mid-September, and anyone found responsible for the deaths of tourists last week could face criminal, civil and administrative penalties.

Police shot dead the hijacker, a disgruntled former policeman, but it was not clear how the hostages died.

The botched rescue, which was broadcast live around the world, drew sharp criticism from China and Hong Kong and Philippine President Benigno Aquino said he had been exasperated by the amateurish attempt.

This is meant to be a full, thorough, comprehensive and credible investigation, de Lima told a news conference.

No whitewash whatsoever. This will be a marathon inquiry.

A five-member inquiry and review committee would begin work on Friday and hoped to be ready in 10 days to submit a report to Aquino and the Hong Kong and Chinese administrations, she said.

At the Quirino Grandstand, where the day-longe siege played out, a memorial service opened with Buddhist prayers and chants.

After a break caused by heavy rain, a Catholic mass followed, with prayers for the victims and a call for justice.

Not only the Chinese are calling for the just and swift investigation of the case, we Filipinos demand the same from our officials, Bishop Broderick Pabillo said.

China's ambassador to the Philippines, Liu Jianchao, said after the service that a fair and thorough investigation would help relieve the anger and loss of Hong Kong and Chinese people.

(Editing by John Mair and Sanjeev Miglani)