Five hostages were brought to the Manila Hospital, two of them dead on arrival and three alive without injuries.
The Manila Doctors Hospital reported it received four dead and one woman in critical condition.
The incident started when a dismissed policeman armed with a M16 rifle seized the busload of Hong Kong tourists to demand his reinstatement in the force.
Police officer Roderick Mariano cited the Filipino driver who escaped moments before police surrounded the bus as saying the hostage-taker, identified as former Senior Inspector Rolando Mendoza, 55, opened fire at the tourists.
The gunman earlier released nine hostages — leaving 15 inside — and demanded his job back to free the rest.
Police were shown by local media jumping into the bus after a tense and lengthy stakeout.
A sniper was said to have shot the hostage-taker as the assault order was given out.
Police used hammers to smash side windows, door and windscreen.
According to newspaper reports from 2008, Mendoza was among five officers charged with robbery, extortion and grave threats after a Manila hotel chef filed a complaint alleging the policemen falsely accused him of using drugs to extort money.
Hong Thai Travel Services Ltd. General Manager Susanna Lau told Hong Kong's Cable TV the tour group had left the Chinese territory of Hong Kong on Aug. 20 for a visit to Manila and was scheduled to fly back to Hong Kong on Monday. She said a Hong Kong tour guide and 20 tourists from the territory — three children and 17 adults — were on the bus. There were also four Filipinos on board.
Mendoza hitched a ride on the bus from the historic walled city of Intramuros and then "declared he is taking the passengers hostage" when it reached Jose Rizal Park alongside Manila Bay, police said.
The area also includes the seaside US Embassy and a number of hotels.
The gunman earlier released nine hostages — leaving 15 inside — and demanded his job back to free the rest.
— Associated Press writers Teresa Cerojano and Min Lee in Hong Kong contributed to this report.
