PORT ARTHUR, Australia - A gunman who slaughtered 32 people at a popular tourist site and nearby pub was captured yesterday by police, after he set fire to a guest cottage where he held three hostages.

The gunman, whom police identified as a 29-year-old with a history of psychological problems, had opened fire with an semiautomatic rifle on tourists at a colonial prison site on the island of Tasmania.

It was the worst shooting massacre in Australia this century.

''Various massacres would pale into insignificance when you look at what has happened in Tasmania,'' said Tasmanian Police Commissioner John Johnson.

The arrest ended a 12-hour stand-off at a bed and breakfast, where the gunman had barricaded himself with the cottage's two owners and a guest.

The hostages' fate was not immediately known, but police said the gunman was the only person they saw inside the house after it was set ablaze.

Among those killed at the Port Arthur historic prison complex were two Canadian tourists and 30 Australians, including several children and a baby, witnesses and police said. One American was among 18 people injured; police said only that the man, from W ashington state, was not badly hurt.

Witnesses said the gunman, a blond surfer-type, had mingled casually in the crowd at the prison complex before pulling a rifle from a tennis bag and shooting methodically at visitors.

He moved on to a local pub, shooting and killing more people, before fleeing to the nearby Seascape bed-and-breakfast cottage at about 5 p.m. Sunday, Australia time.

By early Monday, Australia time, more than 200 local and special police units had surrounded the guest house.