Scottish Labour leader Kezia Dugdale said it was "a barbaric and sickening attack, targeted at young and vulnerable people enjoying a night at a concert" and a "heartbreaking moment for our country".
"A Tory (Conservative) source speaking to the Press Association (news agency) said that the Prime Minister's general election campaign was being suspended", Sky News said.
On Wednesday, Labour allowed local candidates to carry out party leaflet delivery, but banned canvassing or any other activity. But the danger, with only 15 days to go until polling day, was that a prolonged pause would have had a profound effect on the election-allowing the terrorists to boast that they had forced us to change our way of life.
But all sides have agreed that national events - including set-piece speeches by frontbenchers, for example - will not take place until Friday. Their full national campaign will begin again on Friday.
A Conservative spokesman said: "The Conservative party will resume local campaigning for the general election at noon on Thursday, after the minute's silence to remember all those who lost their lives and others who were affected by the callous attack on innocent life in Manchester".
"Resuming democratic debate and campaigning is an essential mark of the country's determination to defend our democracy and the unity that the terrorists have sought to attack".
Britain's opposition Labour Party leader, Jeremy Corbyn speaks at a campaign rally in Liverpool, Britain May 20, 2017.
Prime Minister May is scheduled to meet fellow North Atlantic Treaty Organisation leaders during her visit to Brussels on Thursday before going on to Taormina in Italy for a G7 summit.
Labour confirmed its events planned for Wednesday had been cancelled. Bruter said. Bruter added, "The campaign is being interrupted at a time when the Conservative Party was losing a lot of its lead, so the question is whether that interruption will enable them to erase, if you want, that negative dynamic for them".
Ukip became the first party to say it would resume its national campaign when Paul Nuttall announced he would be unveiling his party's manifesto on Thursday.