National campaigning in the General Election has been suspended in the wake of the terror attack at London Bridge.
Prime Minister Theresa May was chairing a meeting of the Government's emergency Cobra committee in Whitehall on Sunday morning, and was expected to make a statement later in the day.
The Cobra meeting will hear experts' assessment of whether the UK's terror threat level should be returned to "critical" - meaning further attacks could be imminent.
A senior Conservative spokesman said national events were being suspended and a decision on further campaigning would be taken later in the day.
And Labour leader Jeremy Corbyn said his party would also be suspending national campaigning until the evening, after consultation with other parties.
It is the second time that campaigning has been suspended. It was suspended for three days after the Manchester attack on May 22.
The Joint Terrorism Analysis Centre moved the terror threat up to critical following the Manchester bomb, triggering heightened security measures including the deployment of military personnel to support police under Operation Temperer.
The threat level was last weekend lowered to the second highest category of severe, meaning an attack was judged "highly likely".
The Union flag was flying at half mast over Downing Street ahead of the Cobra meeting.
A senior Tory spokesman said: "The Conservative party will not be campaigning nationally today. We will review as the day goes on and as more details of the attack emerge."
And Mr Corbyn said: "The Labour Party will be suspending national campaigning until this evening, after consultations with other parties, as a mark of respect for those who have died and suffered injury."
Liberal Democrats confirmed that their national campaigning was also on hold.
The London Bridge attack, which killed six and injured at least 48, was met by an outpouring of messages from candidates in the election, of sympathy for those affected and praise for the response of the emergency services.
Mrs May said: "Our thoughts are with those who are caught up in these dreadful events."
And Mr Corbyn said: "We are all shocked and horrified by the brutal attacks in London. My thoughts are with the families and friends of those who have died and the many who have been injured. Today, we will all grieve for their loss."
Thanking police, emergency services and NHS staff for their "bravery and professionalism", the Labour leader added: "Those who wish to harm our people, divide our communities and attack our democracy will not succeed.
"We will stand together to defend our common values of solidarity, humanity and justice, and will not allow terrorists to derail our democratic process."
A number of senior politicians urged Mrs May to resist calls to delay the election in response to the tragedy.
Conservative former foreign minister Alistair Burt said: "We don't have a Parliament or MPs at present. If there's no General Election, when would we get one?
"Must carry on. Parliament must be the national forum to decide response. We cannot live by incident and reaction via vox pop and social media."
Conservative Steve Baker said: "We are without a Parliament to scrutinise and legislate until there has been an election. The election must proceed on schedule."
And fellow Tory Peter Heaton-Jones said: "Our campaign is suspended out of respect. But Thursday's election must go ahead."
The Democratic Unionist Party's Ian Paisley Jr called for security to be stepped up at polling stations.
"People must feel safe especially at election time," said Mr Paisley. "Every effort must be made to protect polling stations across the kingdom.
"These terror attacks are attacks on our freedom and on our democracy designed to make us feel fear. Answer back with 'business as usual'. Madness to concede any ground to terror. Stand strong, democracy prevails."
The attack came five days before the election, with opinion polls suggesting great uncertainty about the outcome of the contest.
One new opinion poll by Survation for the Mail on Sunday put Conservatives a single point ahead of Labour, on 40% to 39% for Jeremy Corbyn's party.
However, the findings contrast sharply with a ComRes poll for the Independent and Sunday Mirror which still shows the Conservatives with a 12-point advantage.
Opinium for The Observer also has the Tories ahead but with the gap narrowing to six points.
Liberal Democrat leader Tim Farron said: "Tonight's horrific incidents in London remind us how much we owe our emergency services. My thoughts and prayers with everyone affected."
Scottish National Party leader Nicola Sturgeon said: "Dreadful news from London. My thoughts are with all those affected."
Plaid Cymru's Leanne Wood said: "This is another devastating attack. Plaid Cymru extends its condolences to the people affected and the emergency services that work so hard in these difficult circumstances. People deserve to live their lives without fear, and we must come together to reject hate."
Amid the messages of sympathy and solidarity, there were signs of the terror situation becoming part of the political debate.
Labour's John Mann renewed calls for internet companies to be made liable for the content which their sites host, after long-standing complaints that terrorists are able to use social media to communicate with one another.
Former Ukip leader Nigel Farage said: "We cannot suspend the campaign and normal life indefinitely. We want real action from our leaders, not more hand-wringing."
And ex-Labour MP Simon Danczuk, who is standing as an Independent in Rochdale, said: "Can't believe public will now vote Labour to put Corbyn in Downing Street when he opposes shoot-to-kill and is effectively terrorists' friend."
Sunday morning TV political programmes, which would normally provide a vital forum for parties to push their messages as polling day approaches, were disrupted by the outrage.
The BBC cancelled its pre-election edition of the influential Andrew Marr Show, which had been due to feature election interviews with Mr Farron, Brexit Secretary David Davis and shadow chancellor John McDonnell, as well as Sunday Politics, which was going to feature interviews with Lib Dem former deputy prime minister Nick Clegg, Conservative Treasury minister David Gauke and Labour's Chi Onwurah.
Greens confirmed that they too were suspending national campaigning until the evening, though activities will continue at a local level.
Co-leader Caroline Lucas said: "My heart goes out to all those affected and caught up in these horrific and despicable attacks, and my thoughts are with the victims and their families.
"Deep thanks to our emergency services who have responded again with enormous bravery, and to the many members of the public displaying courage and compassion in the midst of terrible scenes."
Brexit Secretary David Davis said he did not believe it would be legally possible to extend the election campaign by delaying polling day to a later date.
Mr Davis told BBC interviewer Andrew Marr: "I'm not sure it can be legally done. In order to do this, you'd have to have some change in the law and who's going to do that?
"Parliament no longer exists. I'm not a Member of Parliament for the duration, as are none of the other people who were MPs."
Mr Davis said he thought that the election was "locked in" to the scheduled date, and that the public would want that to remain the case.
"On the one hand, clearly we want to respect the people who've been injured and killed and therefore we don't want to carry on across it," he said. "On the other hand, the people doing this are doing it because they despise the freedoms we have, and those freedoms could be the freedom to go out on a Saturday night or the freedom to cast a vote."
Mr Davis said it "may well be" that the attacks were intended to disrupt the election, adding: "In which case, all the more reason not to defer, not to deflect, to as far as possible within the bounds of propriety to not let this pull us off course."
He said he expected the suspension of campaigning to be "relatively brief".