COLOMBO, Sri Lanka (AP) _ Two suicide bombers today detonated a van filled with explosives at the headquarters for government operations against Tamil rebels, killing at least 51 people and wounding 120, military officials said.

The government, whose figures are often conservative, gave a preliminary toll of 20 people killed, most soldiers, and 50 injured.

There was no immediate claim of responsibility, but the government suspected Tamil rebels.

The blast devastated a military barracks and blew the roof off the command center's main office across the street. It left a crater in the street six feet deep and 20 feet in diameter. Hundreds of people ran screaming in panic through the streets of the residential neighborhood in the capital.

Walter Fernando, a deputy secretary in the Defense Ministry, said two men tried to drive into the command center and detonated the explosives when they were stopped by sentries at the gate.

He said at least 155 pounds of plastic explosives were used. At least six sentries and the two men in the van were killed instantly, he said.

Fernando said the Tamil Tiger guerrillas were the main suspects. An earlier government statement blamed the attack on ''separatist terrorists,'' the term it uses for the Tamil Tigers.

''There are no other Tamil separatist terrorists operating in Sri Lanka at the moment except the Tamil Tigers,'' said Bradman Weerakoon, foreign affairs adviser to President Ranasinghe Premadasa.

Sathasivam Krishnakumar, a spokesman for the Liberation Tigers of Tamil Eelam, said he did not know anything about the bombing. He was contacted at the guerrilla group's political office in London.

A senior military official, speaking on condition of anonymity, said 35 or 37 soldiers and at least 16 civilians were killed. He said between 120 and 150 people were injured.

The explosion blew the tile roof off the administrative headquarters of the Joint Operations Command, the government nerve center for the war against the Tigers, who have demonstrated experience in sophisticated bomb-making.

Brig. Daya Wijesekera, who was inside the building at the time, spoke of a ''thundering explosion.''

''Everything started flying around, and the roof sagged and caved in.

''Part of a human body - I believe it was a soldier because of his khaki uniform - fell through the roof. There was silence and then we started picking up the pieces,'' said Wijesekera, chief of the media section of the Joint Operations Command.

The worst damage was at the barracks, which was on the street unprotected by a wall. The administrative headquarters, located in a rambling, colonial- style house, was shielded by a six-foot wall, which had chunks blown out of it by the explosion.

The blast occurred exactly one month after the May 21 suicide-bomb assassination of Indian Prime Minister Rajiv Gandhi, whose death in the south Indian state of Tamil Nadu also has been blamed on the Sri Lankan Tamil rebels.

Indian authorities have arrested a half-dozen Sri Lankan and Indian Tamils on conspiracy charges in relation to the murder of Gandhi, who had tried to broker a peace between the Sri Lankan government and the Tamil rebels in 1987. The rebels reneged on the pact.

The Tamil Tigers launched their campaign for a separate homeland in northeastern Sri Lanka in 1983. More than 17,000 people have died since then in massacres, guerrilla warfare and army offensives.

Last week, a Red Cross official said more than 200 Tamils in two villages were massacred by Sinhalese army troops. Tamil leaders said that was only the latest repression of their ethnic group by the army, which is dominated by the majority Sinhalese.

The island nation has been under a state of emergency for the past two years, except for two brief interludes when it was lifted to facilitate local elections. The state of emergency was renewed Thursday for another month by Parliament, which routinely extends it month-by-month.

Tamils make up 18 percent of Sri Lanka's 16 million people. The Sinhalese comprise 75 percent of the population and control the government.