(Adds details of stolen boat, casualties)

SYDNEY, May 1 (Reuters) - Five people were killed in Sydney Harbour on Thursday when a boat packed with revellers on a nightime joyride and a fishing trawler collided, police said.

The owner of the small cruiser said his vessel, which was carrying 14 people, almost twice its legal capacity, appeared to have been stolen by a group of youths.

An American citizen was among those killed in the accident near Bradley's Head, the U.S. Consulate in Sydney said.

"One was heading out to sea and one was coming in. The trawler was heading out to sea. The small boat was out on a social activity," a police spokesman told a news conference.

The accident brought to 10 the number of people killed in collisions in the busy harbour since March 2007, which have prompted local authorities to consider tighter boating regulations.

Four women and one man, aged from their late teens to early 20s, were killed, and nine people were taken to hospital, two with serious injuries.

Australian media said all the casualties appeared to be from the 23-foot (7-metre) half-cabin cruiser, which was owned by a ship-repair company.

"Our boats would only go on the harbour at night-time if we were called out to an emergency for one of our clients' vessels and that certainly didn't occur last night," boat owner John McPherson told local media.

"It appears that somebody has decided that they were going to take our boat for a joy ride and this horrible event's happened."

Passers-by helped those in the water to reach shore, and rescue boats and helicopters rescued survivors within an hour.

"So much of Sydney's life and activity happens in and around the harbour. People go out to celebrate and have a good time, and for this to be the end of it is just breathtaking," Australian Prime Minister Kevin Rudd said during a speech at the opening of a medical school in Melbourne.

There was no immediate word on why the boats collided. Local fishermen said the harbour was calm overnight, with plenty of light and little wind.

Just over a year ago, four people were killed when a ferry collided with a private boat under Sydney Harbour Bridge. (Reporting by James Grubel; Editing by John Chalmers)

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