The Philippine government is to send a high-level delegation to Hong Kong amid increasing public anger there over the deaths of eight citizens in the hijacking of a tourist bus in Manila.
Officials in both Beijing and Hong Kong have expressed concern over the handling of the seizure.
Government offices in Hong Kong are flying flags at half mast and all tours to the Philippines have been cancelled.
The Philippines has declared Wednesday a day of mourning in solidarity.
Philippine presidential spokesman Ricky Carandang has also appealed for calm following reports of apparent retaliatory actions against Filipinos.
"We understand the anger and the dismay over the people of Hong Kong but at the same time, we don't think it's right that our ordinary citizens who had nothing to do with this should be paying the price," he said.
'Can't accept this'
Philippine president Benigno Aquino has promised a "thorough investigation" into Monday's siege, which began when a former police officer, Rolando Mendoza, hijacked the bus carrying tourists in an apparent attempt to get his job back.
Nine passengers were freed in the course of the stand-off, but survivors and experts have criticised the police for being indecisive and slow in their handling of the hijacking.
In the last hour of the siege, police failed in an attempt to board the bus, being forced back by gunfire from the inside of the vehicle.
Almost one hour later, they managed to get on board the bus. The gunman was killed, and eight of the 15 passengers still on board were also dead.
"The Philippine government ... I can't accept this. Why did they do this to us?" said one of the survivors, identified as Mrs Leung. Her husband and two daughters were killed and her son is in intensive care.
"[The gunman] did not want to kill us. He only shot us after the negotiations failed," she told Cable News TV.
The police commander in the Philippine capital of Manila, Leocadio Santiago, has admitted that mistakes were made.
"We saw some obvious shortcomings in terms of capability and tactics used, or the procedure employed and we are now going to investigate this," he told a local television station.
Officials in both China and Hong Kong are also seeking answers.
"We demand that the Philippine authorities conduct a detailed and comprehensive investigation on the incident. They must provide a full account to us as soon as possible," Hong Kong Chief Executive Donald Tsang said.
Chinese Foreign Minister Yang Jiechi telephoned his Philippine counterpart, Alberto Romulo, to say China "was appalled" by the murders.
A "black" travel warning has been raised in Hong Kong, which urges all travel to the Philippines to be cancelled and calls on Hong Kong people in the Philippines to leave as soon as possible.
A high-level Philippine delegation headed by Mr Romulo will travel to Hong Kong in the next few days to explain what happened, a Philippine spokesman said.
Susanna Lau, general manager of Hong Thai Travel Services which had organised the tourists' trip, told the BBC that representatives had been sent to Manila to help the victims and their families.
She described police actions as disappointing, saying: "They could have done better to save all the lives, but no, we lost eight lives instead."