The children on board the bus are reported to be aged five and below.
"I love these children. If blood will be spilled it will not come from me, it will be from the police outside"
Hostage-taker Jun Ducat, speaking to local radio
The hostage-takers apparently include the owner of a day-care centre in a poor Manila suburb where the incident is believed to have begun.
The man, who gave his name as Jun Ducat, has spoken to local radio saying he wanted to ensure the safety of the children and seeking assurances that the police would not attack the bus.
"I want to go home to my children, I will surrender if I will be given an assurance by politicians that they will make sure these children finish college," Ducat said.
"I love these children. If blood will be spilled it will not come from me, it will be from the police outside."
Ducat is apparently the owner of day-care centre in the suburb of Tondo and said the hostage-taking was for the children's benefit.
"To the parents of the kids I am with ... I am asking for justice so they can have continued education up to college,'' he said.
'A good person'
|Dozens of children remain on the bus [AFP]|
Mothers of some of the hostages have also been interviewed on local radio appealing for their children's safety.
"We are asking him to free the children, to let our kids out,'' said Dema Arroyo, 29, mother of 6-year-old hostage Angelica.
"We will forgive him if he will free our children. We have no ill feelings toward him. He is a good person."
A Philippine senator has joined negotiations and was seen boarding the bus to talk to the gunmen.
Senator Bong Revilla left the vehicle after about 45 minutes carrying a boy who had reportedly developed a fever during the stand-off.
Revilla, who said he knows Ducat, said the other children were in good shape.
He said Ducat was holding a grenade with the pin pulled out, and that his hands were shaking.
Meanwhile Esperanza Cabral, the Philippine social welfare secretary, has reportedly also spoken with Ducat and offered assurances that the children would get a good education.
Television pictures showed the bus parked in the square in front of the city hall with curtains drawn across most of its windows.
Earlier a woman hostage, apparently one of the teachers, was seen through the front of the bus signalling for a phone as one of the men held what appeared to be a grenade at her side.
Police officials say they have been given orders to ensure that the hostage drama, playing out live on Philippine television, ends peacefully.
"Right now negotiations are ongoing," Manila police chief Reynaldo Varilla told DZMM radio. "We hope we can have them surrender peacefully so no one will be hurt."