About 6,000 London police officers are guarding the city's mass transit network Thursday, precisely four weeks after a four-man team of British Muslim suicide bombers killed themselves and 52 passengers on subway trains and a bus.

The police operation includes uniformed officers, specially armed units and undercover agents in street clothes, all trying to prevent a repetition of the deadly bombings that tore through London's transit system on July 7.

The deputy assistant commissioner of London's transport police, Andy Trotter, explained the objectives of the operation during an interview on British radio. "There's no specific intelligence, but London is on a very high state of alert, and it is the [4-week] anniversary of the first attack, and we're out there in large numbers," he said. "We are using all the tactics at our disposal to deter any attack on the system and to make sure that the system runs safely today."

London commuters got back on board the city's reopened Piccadilly Line subway service for the first time since the bombings. The worst-hit carriage blown up on July 7 was on a Piccadilly train traveling between King's Cross and Russell Square stations. The blast killed 27 people.

Commuters say they felt a mixture of sadness and determination as they got back on the Piccadilly line. "It's a bit sad actually, coming from King's Cross to Russell Square," said one woman. "I felt a bit sad, you know, because of what happened; but you know, I'm glad, I'm glad they've opened it up again. Life goes on."

"I admit I was pretty scared," said another woman, "but I mean, it's working fine and I just sort of closed my eyes and tried not to think about it. I mean it's almost like a matter of necessity but I'll try to take overland as much as possible, but I guess people are quite resilient."

London transport officials report a sharp drop-off in passenger numbers since the July 7 attacks, and a second wave of attempted bombings on the transit system that came two weeks later, on July 21.

On weekdays, the decline has averaged between five and 15 percent, while on weekends there are nearly one-third fewer passengers than before the bombings.

In other developments, a London court has remanded into custody the first suspect in the July 21 incidents to face formal legal charges. The man, Ismael Abdurahman, is accused of assisting the escape of one of the prime suspects in the case.

Meanwhile, London police have declined to comment on remarks by New York police that the July 7 London bombs were concocted from easily obtained chemicals such as hair bleach and citric acid, and were triggered by cell phones.

Those details have emerged from a briefing led by New York Police Commissioner Ray Kelly for security officers from some of the American city's major business firms.