Two negotiators had approached the bus and spoken to the gunman who gave the negotiators a list of demands, local television reported.
Mendoza asked for food for the remaining 16 on the bus, which was delivered, and fuel to keep the air-conditioning going.
A handwritten note in bold letters talking of a "big deal" after 3pm (0700 GMT) was posted on the glass door of the bus, television images showed.
The note read: "Big deal will start after 3pm today," but the deadline passed without incident.
Later the note was replaced with another message saying: "Big mistake to correct a big wrong decision."
A Manila police spokesman said Mendoza was co-operating with the authorities and that the use of force would be a last resort.
"They are all safe, no untoward incident has been reported," Isko Moreno, the vice-mayor of Manila, told radio station DZMM.
Police had earlier reported that the tourists were from South Korea but later corrected themselves, saying that most of them were from Hong Kong.
Mendoza hitched a ride on the bus from the historic walled city of Intramuros and then declared he was taking the passengers hostage when the bus reached Jose Rizal Park alongside Manila Bay.
The curtains on the bus windows were drawn and live TV footage showed two police negotiators walking to and from the bus and communicating with Mendoza from the window near the driver's seat.
Police officials said they were also using the driver's cell phone to talk to Mendoza.
According to newspaper reports from 2008, Mendoza was among five police officers who had been 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.
Mendoza's younger brother, Gregorio, also a policeman, said that his brother felt that "injustice was done on him".
"He was disappointed that he did well in police service but was dismissed for a crime he did not do," he said.
In March 2007, not far from Monday's hostage taking, a man took a busload of children and teachers hostage from his day-care centre in Manila to denounce corruption.
They were freed after a 10-hour standoff.