The arrest of two Palestinian brothers suspected of involvement in the West Berlin discotheque bombing and the attempted sabotage of an El Al airliner is being attributed to much improved cooperation among European police agencies to combat international terrorism.

Until Scotland Yard sent an urgent message to West Berlin police late last week, an intensive search for perpetrators of the April 5 explosion at La Belle discotheque had, in the words of one German investigator, "failed to turn up lukewarm clues let alone hot leads" as to who had planted the bomb that killed two persons and injured 230.

U.S. and West German intelligence had determined through radio intercepts that Libya's embassy in East Berlin was aware of plans for the bombing. But West German police officials admitted they had no "concrete indications" of who had committed the act.

But last Thursday, a sharp-eyed security guard at London's Heathrow Airport spotted something unusual about a pregnant woman's suitcase. Police found 10 pounds of plastic explosive concealed inside, and later learned she had been duped by her Arab boyfriend to carry it aboard the El Al flight to Tel Aviv. The bomb was timed to explode in midair and probably would have killed all 400 passengers.

That afternoon, London police released the identity and picture of the boyfriend, who had promised his fiance that he would take a later flight to Israel, where they would be married. He is Nezar Hindawi, a Palestinian from a wealthy and respected family in Jordan.

West German police were notified that Hindawi had a brother in West Berlin, where he might seek refuge. The brother, Ahmed Nawat Mansur Hasi, was tracked to an apartment in West Berlin's Tempelhof district. He had been living in the western sector as a stateless person since 1975, with no record of extremist political activity.

Like thousands of Arab and Asian refugees, Hasi crossed into West Berlin via East Germany's Schoenefeld Airport on a transit visa issued by the communist authorities. To maintain the principle that Berlin should be a free and undivided city, the three allies responsible for security in West Berlin -- the United States, Britain and France -- usually refrain from conducting entrance controls to their sector. But spot checks have begun as part of the extraordinary security precautions undertaken by allied authorities since the disco bombing.

West Berlin police reported today that inside Hasi's apartment, detectives found incriminating materials "which suggest participation in the attack on the discotheque," according to Volker Kaehne, a city judiciary spokesman. Hasi was arrested formally on Sunday after several witnesses at the disco identified him in a police lineup.

West German police officials said the arrest of Hasi was the first major break in the case and could not have come without the British tip-off, which they said was prompted in turn by a new determination among the allies to step up their cross-border collaboration to capture terrorist suspects.

Interior ministers from the 12 European Community countries are to discuss further cooperation at a meeting in The Hague later this week that also will be attended by U.S. Attorney General Edwin Meese III.

West Berlin police have refused to acknowledge whether they know if Hasi maintained frequent contacts with the Libyan People's Bureau in East Berlin. A judiciary official said there was some information suggesting a connection with Libya, but that Hasi appeared for now to be an accessory to the bombing rather than one of its masterminds.

Police spokesmen denied newspaper reports that documents found in Hasi's apartment included plans for further bomb attacks or sketches of potential targets.

Meanwhile, in London today, Hindawi appeared in court amid tight security to face charges of attempting to blow up an aircraft and conspiring to murder his girlfriend, Anne Marion Murphy, a chambermaid at London's Hilton Hotel.

The charges brought forward by Chris Bird, the chief detective inspector of the city's antiterrorist unit, declared that Hindawi conspired on "diverse dates, between Feb. 12 and April 17, in London and elsewhere" with unknown figures to carry out the attack.

The dates indicate that planning for the El Al bombing may have begun well in advance of the U.S. military strike against Libya on April 15.

Bird asked that Hindawi be held in custody until May 1 when evidence of his role would be presented. The magistrate rejected a defense plea for bail after objections from the inspector, who cited the serious nature of the charges and the risk that Hindawi might flee.

Police said a second brother of Hindawi, whose name was given as Mahmoud, has been taken into protective custody in London. He has not been charged with any crime and appears to have aided the police in arresting his brother a day after the bomb was discovered at Heathrow. Washington Post London correspondent Karen DeYoung contributed to this report.