Tensions among security authorities surfaced here today as a police inquiry began into yesterday's bombing of the Grand Hotel in Brighton. The Irish Republican Army claims responsibility for the blast that nearly took the life of Prime Minister Margaret Thatcher and most of her Cabinet.

Police, searching for suspects, detained a man at the airport in Liverpool under the Prevention of Terrorism Act and took him to Brighton for questioning, United Press International reported.

As Thatcher celebrated her 59th birthday today at her country home outside London and Britons hailed the calm and courage she displayed in the aftermath of the blast, there appeared to be popular and press unanimity that the cherished openness of British political life -- as compared to the heavy security surrounding American leaders -- must not be sacrificed.

But questions remained about how the bomb could have been planted and about whether the British may have underestimated the IRA as a terrorist organization.

At a press conference in Brighton today, the chief of Scotland Yard's antiterrorist group, Bill Hucklesby, said the bomb that blew up the hotel headquarters for the Conservative Party convention could have been planted weeks earlier by the Provisional IRA.

"The IRA now have the ability to time a device in a far more sophisticated manner than before. It could be possible to place the bomb and time it to go off in 3 weeks, 2 hours and 30 seconds," he said.

But unnamed security sources, according to the British news agency Reuter, doubted that suggestion and were quoted as saying, "It's more likely that the police are covering up the inadequacy of their own security."

While security was relatively tight at the convention center where the annual party conference was held, it was much less so at the big and very busy hotel next door where Thatcher and many leading government and party officials were staying.

Policemen occasionally checked to see if people were wearing credential badges as they entered the hotel, but there was no inspection of briefcases or anything else. There was tight security around Thatcher's first-floor suite, but police authorities say the bomb probably was planted on the fifth or sixth floor and designed to blow a huge hole through the two floors above and cause the blast and debris to collapse the floors below.

Sussex Police Chief Roger Birch has said that while "all reasonable precautions" had been taken, total security in a place as open and public as the Grand Hotel was "impossible." Nevertheless, The Times of London today described the situation as "the worst security blunder for many years" and the Daily Mirror called it "criminally complacent."

Hucklesby said that police dogs trained to sniff out explosives had gone through the 185-room, seven-story seaside hotel before the conference began but that the IRA may now be wrapping the bombs in cellophane to seal in any odor and may have planted the bomb under floorboards or in a cavity between the fifth and sixth stories.

A fourth body was recovered from the rubble today. At least 34 persons were injured, several seriously.

Police confirmed tonight that Roberta Wakeham, wife of the government's chief whip in the House of Commons, was one of those who died. Her husband remains hospitalized in serious condition.

The blast brings to 80 the number of persons killed, with about 1,000 injured, in IRA attacks on the British mainland during the past 12 years. The attack on the hotel, however, was unprecedented in scope and was the first attempt to assassinate the entire top level of the British government.

Today the British press rang both with condemnation of the IRA terrorists and determination to maintain public access to political leaders.

"If the parties are driven to shrink inside police-ringed islands of security and unreality, then the planters of terror this week will have left their enduring mark. They will have pushed the motors of democracy still further away from the people," said the left-of-center Manchester Guardian in an editorial.

"A liberal society cannot tolerate such stringent security that its leaders are denied access to the people and the people denied access to the leaders," said the right-of-center Times editorially.

The British, in common with most European countries, tend to disdain the extraordinary security that surrounds an American president. But an American security specialist, Jack McGeorge, interviewed on BBC television last night, said that it works, in at least one important way.

American presidents tend to be threatened by individuals with emotional or mental problems, he said, and such individuals tend not to either notice or be dissuaded by heavy professional security. But the more expert terrorist groups, such as the IRA, do notice the heavy security -- both that which is intended to be obvious and that meant to be more subtle -- and may be deterred by it.

So while the British may not like the American style, it is probably worth considering, he said.

The issue of overall IRA effectiveness was raised directly today in Northern Ireland by James Molyneaux, a leader of the Official Unionist Party and a member of the British Parliament. He told the British Press Association that the bombing finally might persuade the British public that the IRA is among the most sophisticated terror groups in the world and not merely "a bunch of wild Irishmen."

Two weeks ago, after the Irish Navy seized a trawler filled with several tons of weapons bound for the IRA, Molyneaux called the capture "in many ways more worrying than comforting. It shows the scale of ambition of the Provisional IRA."