Prime Minister Theresa May and Labour leader Jeremy Corbyn had suspended campaigning after Monday's bombing at a Manchester pop concert, which killed 22 people, including many teenagers, and wounded dozens more.
Eight suspects are currently in detention on UK soil in connection with the blast, for which the ISIS group has claimed responsibility, while police in Libya have detained the father and brother of 22-year-old suicide bomber Salman Abedi.
Washington's top diplomat Rex Tillerson is due to visit London on Friday in an expression of solidarity, after Britain reacted furiously to leaks of sensitive details about the investigation to US media.
US President Donald Trump threatened to prosecute those responsible for the "deeply troubling" security breach, which has strained the close relationship between Washington and London.
At a summit of NATO allies in Brussels on Thursday, May confronted Trump over the issue, saying shared intelligence "should be kept secure," her spokesman said.
Monday's bombing was the latest in a series of ISIS-claimed attacks in Europe that have coincided with an offensive on the jihadist group in Syria and Iraq by US, British and other Western forces.
Britain's terror threat assessment has been hiked to "critical", the highest level, meaning an attack is considered imminent.
Armed troops have also been sent to guard important sites, an unusual sight in mainland Britain, while armed police are now patrolling the country's trains for the first time.
At the launch of the UK Independence Party's manifesto on Thursday, deputy leader Suzanne Evans said May "must bear some responsibility" for the terror attack in Manchester this week due to policing budget cuts.
Opposition leader Corbyn in a speech in London later on Friday is expected to say it is the "responsibility" of governments to minimise the risk of terror by giving police the funding they need.
A YouGov poll published in Friday's edition of The Times put Conservatives on 43 percent compared to Labour on 38 percent, far better for Labour than the double-digit margin that had previously separately it from the ruling party.
YouGov polled 2,052 people on Wednesday and Thursday.
But analysts said that the Conservative prime minister -- who previously served as interior minister for six years -- could benefit at the polls from the shift in focus ahead of the general election on June 8.
"If security and terrorism become more prominent then I can only see one winner from this -- Theresa May," said Steven Fielding, a professor of politics at the University of Nottingham.
At the G7 summit in Sicily on Friday, May will emphasise the importance of tackling the spread of terror online by urging internet companies to do more to remove extremist content.
The premier's visit to Italy has been cut short following the attack in Britain, where police are continuing their manhunt for the bomber's accomplices.
Officers briefly evacuated residents from an area in Wigan, a town near Manchester, on Thursday as they searched a house in connection with the probe and a bomb disposal unit was deployed.
Residents were later allowed to return.
Manchester-born Abedi, a university dropout, grew up in a Libyan family that reportedly fled to the northwestern English city to escape the now-fallen regime of Libyan dictator Moamer Kadhafi.
Libyan officials said he and his brother Hashem belonged to the Islamic State group, while their father Ramadan once belonged to a now-disbanded militant group with alleged ties to Al-Qaeda.
Libya said it was working closely with Britain to identify possible "terrorist networks" involved, while a British official said that Abedi had been on the intelligence radar before the massacre.
On Friday evening a defiant Manchester is set to go ahead with hosting an athletics contest, the Great City Games, in which the likes of former world 100 metres champion Kim Collins are due to compete amid robust security.
Following a minute's silence in the city's St Ann's Square on Thursday, crowds broke into a spontaneous rendition of "Don't Look Back in Anger" by the city's own Britpop band Oasis.
Queen Elizabeth II also visited a Manchester hospital to meet children injured in the "very wicked" attack, which happened at the end of a concert by US pop star Ariana Grande.
Some 75 people were still being treated in hospital, including 23 in critical condition, medical officials said.
Twelve of those injured were under 16, and the youngest killed was an eight-year-old girl.
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