LONDON, JULY 20 -- A bomb exploded at the London Stock Exchange today, striking at the heart of Britain's financial establishment in the latest of a series of attacks on the British mainland that police say was clearly the work of the Irish Republican Army.

The bombing apparently was designed as a symbolic blow since a caller gave 40 minutes' warning to evacuate the area and no one was injured. Most trading, which is done largely via computers and telephones, went on uninterrupted.

The blast tore a 10-foot-wide hole in the modern 23-story building, showering the street with debris and wrecking the public gallery. It also snarled traffic and disrupted business throughout the morning in the City, London's bustling financial center, and sent a clear message about terrorists' ability to strike at civilian targets.

No one claimed immediate responsibility for the blast, but police officials said it clearly had been perpetrated by the IRA, which has waged a 20-year war to end British rule in Northern Ireland.

It was the ninth IRA attack in England this year in a campaign that until now has been limited to military and political targets. Last month, IRA operatives bombed the Carlton Club, a meeting place for leaders of Prime Minister Margaret Thatcher's Conservative Party. About 20 people were injured in that blast, which unlike today's was not preceded by a warning.

Thatcher said she was "appalled" by the attack but grateful that no one was injured. "All of us have got to be very vigilant," said David Waddington, the cabinet minister in charge of police affairs.

A man with a strong Irish accent phoned the Reuter news agency just after 8 a.m. to warn about the blast. The same caller then phoned an emergency services line and the stock exchange itself, making eight calls within 18 minutes and repeating his warning, according to police. In all the calls, the man used an IRA code word, known by police, to demonstrate his authenticity.

Police evacuated about 300 people from the building and nearby sites. The bomb went off at about 8:50 a.m.

"It seemed to be a big bomb," said an unidentified broker. "You could hear it from 100 yards down the road and you could also see the smoke from the trading floor."

Asked if there had been any panic inside the exchange building, one worker replied: "Good Lord, no; we're British."

George Churchill-Coleman, commander of the police anti-terrorist squad, told reporters the bomb apparently had been planted in the men's room of the public viewing gallery. He said up to 10 pounds of high explosives had been used.

Churchill-Coleman said it was the first time IRA attackers had phoned a warning since the latest bombing campaign began. He offered no explanation, but analysts said the organization apparently wanted to reap the propaganda gain of damaging an important British symbol without being blamed for civilian casualties. Without the warning, police said, dozens of people could have been killed or injured.

"Clearly every terrorist organization thrives on two things -- fear and publicity," said Churchill-Coleman. "This is what it's moved to today; tomorrow it could be somewhere else."

He said the bombing campaign was the work of small groups of IRA operatives, known as "active service units," that have been staging attacks in England for the past 18 months.

Churchill-Coleman appealed for public help in tracking them down and announced a new hotline phone number to handle calls from tipsters.

"These people are here; they've been here for 18 months and we need to find out where they are living," said Churchill-Coleman. "Somewhere there is somebody who knows where these individuals are."

Analysts say the IRA began shifting operations to England in 1988, in part because attacks here are often easier to launch than in Northern Ireland, where security forces are vigilant. The other reason is impact: Attacks in England invariably draw more public attention and larger headlines in the British press than attacks elsewhere.

IRA operatives have managed to keep security forces off balance by varying their targets and methods of attack. Since May they have killed one British army recruiting sergeant with a car bomb, shot dead an unarmed recruit and seriously wounded two others at a train station, blown up an artillery company headquarters in London and bombed the empty former home of a retired Conservative Party treasurer.

The worst attack in recent years was last fall's bombing of a military barracks in the southeastern town of Deal, which killed a dozen soldiers and wounded many others. None of the bombers has been caught. ATTACKS THIS YEAR LINKED TO IRA

Yesterday: A bomb ripped a hole in the walls of the London Stock Exchange. Police cleared the building before the blast.

June 25: A bomb explosion injured more than 20 people at the Carlton Club, a gathering place in London for the ruling Conservative Party.

June 20: Two British army sergeants were injured when a bomb ripped through a military van in Leicester in central England.

June 14: A bomb blast damaged buildings at a British army training center near Hameln, West Germany.

June 9: A bomb explosion at a military barracks in London injured 17 people. Police said the attack had "all the hallmarks of the IRA."

June 1: IRA gunmen shot and killed a soldier and wounded two others waiting for a train in the central English town of Lichfield. On the same day in Dortmund, West Germany, gunmen killed a British army major outside his home and wounded a police officer who gave chase.

May 27: Two Australian lawyers on vacation in Roermond, Holland, were ambushed and shot by gunmen. An IRA statement said the victims were mistaken for British army personnel.

May 16: A bomb placed under an unmarked military van outside a London recruiting center killed an army sergeant.

May 14: A bomb blast outside the Institute of Army Education near London wounded five people.

April 9: A bomb detonated on a rural road in Northern Ireland killed four members of the predominately Protestant Ulster Defense Regiment.

Feb. 25: A bomb explosion at an army recruiting center in Halifax caused extensive damage but no injuries.

Feb. 20: Three people were injured when IRA guerrillas detonated a bomb beneath a British army truck in the city of Leicester.

Feb. 11: A British Army helicopter in northern Ireland made a forced landing in which three soldiers were injured. The IRA claimed it had shot down the aircraft.

Feb. 7: A bomb blast at the Shorts aircraft factory in Belfast damaged a Royal Air Force training plane.

Jan. 16: A 1,000-pound bomb demolished an unmanned police station in the Northern Ireland border village of Sion Mills.

SOURCES: The Washington Post, Associated Press, Reuter

Compiled by James Schwartz