The UK is at no greater risk now from a terror attack than it was before the Madrid bombings or even 11 September, Home Secretary David Blunkett has said.
Police have launched a poster campaign for Tube stations
Mr Blunkett urged people to be "alert but not alarmed", although it was "quite likely" a terror attack was being planned now against the UK.
His comments came as plain clothes anti-terror police began patrolling London's public transport system.
British Transport Police also plan more random searches for Tube passengers.
A spokesman stressed the moves had been planned for some time and were not a reaction to the attacks in Madrid.
'Don't give up'
Amid speculation that the Spanish blasts meant al-Qaeda had directly struck western Europe for the first time, Mr Blunkett said the terror risk had been high for a long time.
He told BBC News: "We are at no greater risk now than we were before the World Trade Center attack, because they had already planned for it, before the Afghanistan war, before the Iraqi conflict and before last Thursday."
Ministers had stressed they could not guarantee there would never be an attack, he said, adding: "It is quite likely they are planning one now."
But security services were on alert and there was a hotline for members of the public to call.
"I ask, fearful as people are, that they stand with us because there is nothing more likely to erode our freedom and democracy than giving up and being compliant with people who are threatening the very livelihood we have built up in this country," he said.
An emergency meeting of European Union interior ministers is apparently being planned for Friday.
Mr Blunkett said he was prepared to attend such talks, but added: "I don't want to rush into a meeting ill-planned without an outcome."
Meanwhile, three counter-terrorism officers from the Metropolitan Police will travel to Spain to hold talks with their Spanish counterparts.
The move is part of an intelligence exchange and the officers will not be involved in investigating the Madrid bombings.
In London, posters are being put up to encourage people to report anything unusual to police.
Metropolitan Police Deputy Chief Constable Andy Trotter emphasised no specific threat to the London Underground or rail system had been received.
But he said London was on high alert and passenger vigilance was "crucial".
"With three million travelling every day, they can provide millions of eyes and ears across the whole system", he told BBC News, urging travellers to call police if they see "anything suspicious".
The Met's ongoing review of Tube security had been "thrown into stark relief by those awful incidents in Madrid", he added.
"We are throwing everything we've got at this because it is the number one challenge for us."
A British Transport Police (BTP) spokesman said extra patrols, using plain clothes and uniformed officers, were being carried out at Tube stations and on trains.
But there are fears this could leave the BTP overstretched.
BTP Federation chairman Alex Robertson told BBC News an extra 200 staff across the UK were needed to cope with the increased threat of terrorism.
The BTP has also announced a Tube poster campaign, urging people to keep an eye out for unattended bags.
Defence Secretary Geoff Hoon said 14 "rapid reaction groups" of reservist troops had been made available to help civil authorities "at very short notice" in a crisis.
UK rail regulator Tom Winsor said some security measures had been taken, but there was always a danger with land transport as explosive devices could be placed on tracks.