NAGANO, Japan -- A radical left-wing group took responsibility Thursday for a rocket attack at Tokyo's main international airport, and it mocked security measures in place for the Winter Olympics.

The group, calling itself the Revolutionary Workers Association, said it opposes construction of a second runway at Narita airport, 40 miles east of Tokyo. Three homemade rockets were fired into a cargo plane area Monday night, injuring one airport worker. Two of the projectiles exploded.

"We defeated the heavy, counter-revolutionary security for the Olympics," the group said in a statement signed "The Revolutionary Army."

"We are determined to continue fighting guerrilla wars to prevent further construction at the airport and to eventually close it," said the letter, a copy of which was provided by police. Leftist groups claim a larger airport could be used for military purposes.

THE GROUP ALSO SAID SECURITY guidelines updated last year between Washington and Tokyo "could lead Japan to war and fascism." The guidelines commit Japan to greater cooperation with the U.S. military.

Left-wing radicals have long opposed the expansion of the airport, which opened with only one runway in 1978.

The attack came as athletes, officials and spectators were arriving for the Olympics. Police said they had no evidence the rockets were aimed at disrupting the games, which start Saturday.

Police are out in force in Nagano to protect the Olympics. With the games set to open Saturday, pressure on the police has increased significantly because of the airport attack.

"We will do our best to ensure the safety of dignitaries from home and from abroad and tighten security at all facilities," Yuko Sekiguchi, head of the National Police Agency, was quoted as saying Thursday by the Kyodo news service.

THE AGENCY SET UP A SPECIAL force in Tokyo last month to strengthen security during the Olympics and urged local police to keep a close eye on leftists. The agency also suggested that major airports tighten security during the games.

The Nagano Games are to feature more athletes (2,450) from more countries (72) than any previous Winter Olympics. More than 1 million tickets to Olympic events have been sold, and the games are to be seen on television in as many as 160 countries.

This year also marks the Winter Olympics' return to Asia after a 26-year absence. Any incident would be a deep embarrassment to Japan's government, which prides itself on the relative safety of its cities.

To avert any surprises, 6,000 police - including 4,000 reinforcements from across the country - are being mobilized for the games in Nagano, about 115 miles northwest of Tokyo. Another 3,000 security personnel are working for the organizing committee itself.

"Now is the peak of the arrival of the IOC family, and athletes and officials so we have been reinforcing and increasing our security measures," committee spokesman Ko Yamaguchi said Thursday.

HE SAID THE TIGHTENED SECURITY was not in response to any specific threat, however.

Olympic officials said they're confident in the steps taken to protect the games.

Kevan Gosper, a member of the IOC's executive board, said the IOC has been told by Japanese authorities that the airport incident was unrelated to the Olympics.

Even so, local police doubled the number of officers at the airport in Narita, about 40 miles northeast of Tokyo, from 500 to 1,000 beginning in late January, said spokesman Akihiro Adachi. They have also set up an 80-person force to investigate the attack.

The higher police profile was apparent in Nagano as well.

Swarms of police dressed in blue baseball caps and ski parkas gathered Wednesday at Olympic venues to practice their security plans before the games begin.

PATROLS ON NAGANO'S MAIN STREET also appeared to have increased. At the end of the street is one of Japan's oldest Buddhist temples. Police have said they are concerned that the wooden structure could be vulnerable to arson.

A major focus will be the arrival of Emperor Akihito, Empress Michiko and other VIPs from around the world for the opening ceremony. Armored cars regularly patrol near the stadium where the ceremony will take place.

Leftists strongly oppose the Japanese monarchy, but attacks on it have been rare since the death of Emperor Hirohito, Akihito's father, almost 10 years ago.

Matsumoto, a city just south of Nagano, was the site of a nerve gas attack that killed seven people three years ago. The attack was carried out by the Aum Shinri Kyo cult, the same group that gassed Tokyo's subways in 1996, killing a dozen people. The cult has since been disbanded and most of its leaders are in jail.

The only reported security scare in Nagano so far has been a suspicious package that arrived at the organizers' headquarters last week. A police bomb squad was mobilized, but the parcel contained a toilet seat warmer, gloves and hand warmers.