TOKYO (AP) _ Frogmen in moats, sentries aboard blimps and phalanxes of riot police will form part of an estimated 30,000-man security force for the summit meeting of seven Western industrial nations May 4-6 in Tokyo.

It promises to be Japan's largest security operation.

President Reagan will be at the meeting with the leaders of Japan, Britain, Canada, France, Italy and West Germany.

The summit, coming between the 60th anniversary of Emperor Hirohito's reign and a visit by Britain's Prince Charles and Princess Diana, is likely to tax Japan's security forces as never before.

''In one way or another, it will involve every officer in our 250,000- force,' ' said a National Police Agency official, referring to widespread house-to-house inspections, detailed checks of all gun owners and users of explosives and dangerous chemicals.

''The situation is more severe than the 1979 Tokyo Summit,'' said Shinji Suda, an official of the Metropolitan Police Department's Security Division. ''Japanese radicals and international terrorists have acquired new techniques and can lay hands on newer weapons.''

The metropolitan police used about 25,000 officers a day on guard duty during the 1979 summit.

Authorities say about 30 minor radical groups of various kinds have threatened some types of action during the summit, but the primary worry for police is the leftist Chukakuha, or Middle Core Faction.

In past operations, this Marxist militant group, with an estimated 1,300 members, fired home-made missiles across runways at the International Airport at Narita and damaged the governing Liberal Democratic Party's headquarters with homemade flamethrowers fired by a sophisticated timing device.

In November, Chukakuha members claiming to support striking railway workers paralyzed Tokyo's railroad network by severing computer, communications and signal cables at 20 locations in coordinated pre-dawn attacks, tying up about 6 million commuters.

On March 25, terrorists fired five homemade rockets from stolen cars at the the U.S. Embassy and the Imperial Palace. Although the crude projectiles caused no damage or injuries, the attackers' ability to slip stolen cars with crude missiles concealed inside through already tight security was of special concern to authorities, Japanese news reports said.

The terrorists used sophisticated timing devices to open the cars' trunks and fire the rockets, according to one newspaper report, quoting investigators.

Although no organization claimed responsibility, authorities said they suspected one of the radical leftist groups was responsible. Police searched the headquarters of the small Senki, or ''Battle Flag'' faction, a rival to the Middle Core group.

''We are also closely watching infiltration attempts by the Irish Republican Army, Germany's Baader-Meinhof gang, the Italian Red Brigades, and Abu Nidal and other Arab terrorists,'' said the National Police Agency official, who spoke only on condition of anonymity.

The security machine, in the planning stages for eight months, will start up April 28, when radicals are expected to stage annual demonstrations on ''Okinawa Anti-War Day.'' This marks the anniversary of the San Francisco Treaty of April 28, 1952, that formally ended World War II in the Pacific and officially turned Okinawa over to U.S. forces that had occupied it since 1945. Okinawa reverted to Japan in 1972.

On the emperor's 85th birthday April 29, a ceremony also will mark the 60th anniversary of his reign. While the emperor normally is not a radicals' target, some, including the Middle Core Faction, have threatened to disrupt the imperial ceremonies, police said.

Prince Charles and his wife are due May 8 for an eight-day state visit and security forces will work through that.

In addition to Reagan and Japan's Prime Minister Yasuhiro Nakasone, the other leaders coming for the summit are President Francois Mitterrand of France, Prime Minister Margaret Thatcher of Britain, Chancellor Helmut Kohl of West Germany, Premier Bettino Craxi of Italy and Prime Minister Brian Mulroney of Canada. Premier Rude Lubbers of the Netherlands has been invited as an observer for the European Common Market.

National police officials said summit security includes special helicopter units and a blimp, 15,000 riot police and 250 police martial arts experts and marksmen to act as bodyguards for visiting officials, all of whom will have their own security as well.

Metropolitan police frogmen will patrol 17th-century moats near the Akasaka Palace, the state guest house which will be the venue for the three-day talks, and around the Imperial Palace. Officers also will open, inspect and reseal sewers daily in Akasaka and around hotels used by visiting delegations. Traffic in central Tokyo will be tightly restricted.

Except for Reagan and Mrs. Thatcher, who will stay at their governments' ambassadorial residences, the leaders will be quartered at the Hotel New Otani.

Buildings facing the New Otani and the Akasaka Palace will be checked.

''We will keep an eye on buildings and lots within 1.5 kilometers (about one mile) from these buildings and roads in between,'' Suda said. ''We know that the range of their (terrorists') missiles is about one kilometer (0.6 miles).''

In an apparent move to discourage terrorists, Security Police drills were shown on national television in mid-March. Viewers saw one officer use judo on knife-wielding ''attackers'' while marksmen fired on distant shadows and ''VIPs'' were quickly evacuated in helicopters.