Security forces were on high alert in London [Images] with undercover officers mingling with people as the tube network (underground railway) fully reopened on Thursday, four weeks after the deadly terror attacks on the transport system which killed 56 people.

Over 6,000 personnel patrolled the streets, buses and trains, seeking to restore confidence in nervous commuters. Metropolitan Police Assistant Commissioner Tarique Ghaffur said extra officers had been brought in from outside the capital and retired officers had been persuaded to return to help with the anti-terror work.

Though there was no specific intelligence about another attack, Detective Chief Constable Trotter said, "This is the biggest threat London has faced in peacetime and we have to throw all our resources into it right now. We will continue as long as it is necessary."

Multiple blasts rock London

'We will explore every opportunity we have. Any help we are able to call upon, we will call upon,' Ghaffur told BBC.

London Underground said all stations on the tube network had been opened, with the Piccadilly Line reopening and the Circle Line beginning a limited service. The limited shuttle service is running for the first time in Circle Line, the last Tube line still affected by the  July 7 attacks -- exactly four weeks on.

"It's a very poignant moment today," said Trotter. "Today is part of getting London back to normality. It will be a different normality from before -- the level of security will have to change for a long, long time to come. Obviously people are nervous and anxious, but they are happy to see us."

Meanwhile, the first person charged in connection with the botched attacks of July 21 was today remanded to custody and was expected to appear in court on August 11. Ismael Abdurahman, 23, had been charged under UK anti-terror laws, for withholding information that may have helped catch people involved in the July 21 attacks.

In Italy [Images], the date for extradition hearing for London bomb suspect Hussein Osman has been scheduled for August 17. Osman is one of the four bombers of July 21, the only one to be arrested from abroad. The other three suspects are already in custody. Osman has told Italian magistrates that he took part in the failed July 21 attacks, but that he did not intend to kill anybody. His extradition process should be completed by early December.