Flags flew at half mast and leaders observed three minutes of silence Thursday as Hong Kong mourned the 38 victims of a ferry collision that sent shockwaves through the Asian financial centre.

Chief Executive Leung Chun-ying led senior officials in a memorial service at the harbourside government headquarters, while schools and other public institutions also fell silent.

Hong Kong's worst maritime accident in 40 years saw a high-speed ferry, the Sea Smooth, collide with a pleasure craft, the Lamma IV, carrying around 120 passengers on a company trip to watch national day fireworks on Monday night.

The Lamma IV's left rear was torn open in the impact, throwing scores of passengers into the sea. The vessel's stern flooded within minutes, trapping passengers in the submerged cabin.

British Prime Minister David Cameron sent his condolences to the victims after the consulate in the former British colony confirmed that an unidentified Briton was among the dead.

US Consul General Stephen Young and European Union foreign policy chief Catherine Ashton also extended condolences, with Ashton saying the accident was "a serious blow to a city for which the sea is its soul".

Mourners accompanied by Taoist priests boarded boats in a sad procession to the scene of the collision off Lamma island, a few kilometres (miles) southwest of Hong Kong, where they threw paper offerings to the dead into the sea.

Shock and disbelief that such an accident could have happened in one of the world's busiest ports, which prides itself on its state-of-the-art transport infrastructure, gave way to grief as the traditional mourning period began.

Around 500 people attended a memorial service at St John's Cathedral, where Hong Kong Electric executives paid tribute to staff who died on the Lamma IV in what was meant to be a holiday cruise for employees and their families.

Hong Kong Electric is part of the business empire of Asia's richest man, Li Ka-shing, who offered HK$200,000 ($25,800) to the families of each of the deceased.

Hong Kong Electric group treasurer Vincent Chow said he had lost a "fantastic" colleague who had worked for the company for 30 years.

"It came as a total shock. We never thought it could happen," he told AFP at the church.

Pharmacist Katherine Chow was one of many members of the general public who attended the service.

"No one anticipated this kind of disaster. No one," she said.

She said she did not know any of the dead, but wanted to attend the service "because I'm part of Hong Kong -- the disaster bonds everyone together".

Passengers on both vessels described scenes of panic and chaos after the collision. As the pleasure craft partially sank, the Sea Smooth catamaran limped to port on Lamma, taking on water through a gaping hole in its left bow.

Investigators pored over the salvaged wreck of the Lamma IV seeking clues as to how two large, seaworthy vessels could have crashed into each other on a clear and relatively calm night.

Police arrested the captains of both vessels on Tuesday along with five crew, pointing to possible human error as the cause of the accident.

Criticism has also been levelled at ferry operators for ramping up the frequency of services during the holiday period, placing unreasonable strains on captains and crew.

Authorities have said that in a six-month probe, investigators will try to determine why the Lamma IV sank so quickly, whether there was adequate safety equipment on board and if the captains followed the rules of the sea.

Ferry operator Hong Kong and Kowloon Ferry Holdings (HKKF) said its captain was a 20-year veteran and rejected allegations from Hong Kong Electric that he abandoned the sinking pleasure craft.

HKKF general manager Nelson Ng said the captain had been discharged from hospital and had given a statement to police, but had not otherwise spoken about what happened.

"Regardless who is right and who is wrong, it has cost many lives. Only the captain himself knows best what actually happened... but the crew emphasised they did not hit-and-run," he told public broadcaster RTHK.

At least five children were killed in the collision. More than 100 people were injured, including some who remain in critical condition in hospital.