Some 6,000 police officers are patrolling streets of London and the capital's subway system a month after four bombs killed 52 people.
The massive security operation is intended to reassure the public four weeks on from the deadly July 7 attacks on three subway trains and a central London bus, and the failed July 21 attacks that followed.
Officials stressed there was no specific intelligence of a third attack, but undercover police were mingling with passengers, and officers were armed with automatic rifles and pistols. Police helicopters hovered above while traffic was heavier than normal.
"It's a little bit eerie," said Rosalyn Cooper, 23, before she got on the Tube. "You can't help thinking about when it will happen again."
London transport authorities, meanwhile, opened the subway line worst-hit in the July 7 attacks, easing disruption on the network that carries some 3 million passengers a day.
On the Northern line, authorities told commuters to watch out for suspicious activity.
"This is a major threat and we are doing everything we can, both covert and undercover policing, to try to stop another attack," said Deputy Chief Constable Andy Trotter of the British Transport Police. "There is no specific intelligence but we are still at a very high level of alert in London," he added. "It's four weeks on from the first attack and this operation today is to reassure Londoners."
The Piccadilly Line, which reopened Thursday, is one of the busiest sections of London's subway system. It had been partly suspended since a bomb exploded on a train in the July 7 attacks, killing 21 commuters - a closure that caused significant disruption on the network.
"The return of the Piccadilly line is a major step as the Underground and London get back to normal," London Underground Managing Director Tim O'Toole said.
Transport for London, the public body which runs the Underground, said passenger numbers had dipped by 30 percent at weekends and by between 5 percent and 15 percent on weekdays since July 7.
Meanwhile, almost 8 million pounds (US$14.2 milllion; Ђ11.5 million) has been raised through charitable donations to help victims of the July 7 attacks. The London Bombings Relief Charitable Fund said bereaved relatives would receive payments of 5,000 pounds (US$8,900; Ђ7,200), while people who were injured and spent a night in hospital will receive 3,000 pounds (US$5,300; Ђ4,300).
The first man charged in connection with the London bombings was to appear in court Thursday as police continue to question another 14 suspects in custody on suspicion of involvement in the July 21 attacks.
Ismael Abdurahman, 23, from southeast London, was to appear at Bow Street Magistrates' Court in central London to face charges of withholding information about those who carried out the failed attacks.
Prosecutors allege that Abdurahman, who was charged under anti-terrorism legislation, had information he knew might help police capture suspects involved in "the commission, preparation or instigation of an act of terrorism."
One of the suspected bombers, Hamdi Issac, has been charged in Italy with association with the aim of international terrorism. He traveled to Italy shortly after the botched attacks and Britain is seeking to extradite him.
Britain is holding 14 suspects in connection with the investigation, and Italy has three. Police believe they have all the July 21 bombers in custody.
Italian prosecutor Pietro Saviotti said Issac's extradition could be delayed by some weeks because local authorities were looking into possible crimes committed in Italy. But he dismissed suggestions that it would block the transfer.
"I would not say we are talking about days, but about weeks" before Issac can be extradited, Saviotti said.
Police were also pursuing international links - to Pakistan, Saudi Arabia and Zambia - as they hunted for possible conspirators in the bombings and tried to determine whether the two sets of attackers were linked. No suspects are being held in connection with the deadly July 7 suicide attacks, the AP reports.