London reacted furiously after sensitive details about the probe into Monday's suicide attack, which targeted young concert goers and killed 22 people, appeared in the US press.
And as more children were named as victims of the massacre, police carried out fresh arrests and raids.
With the row over intelligence-sharing escalating, a shellshocked Britain held a minute's silence to remember the victims of the latest Islamic State-claimed atrocity to hit Europe.
After bowing their heads in silence, the grieving crowd in Manchester's St Ann's Square broke into a spontaneous, gentle rendition of "Don't Look Back in Anger" by the city's own Britpop band Oasis.
It was a message of defiance three days after Manchester-born Salman Abedi's attack on young fans attending a concert by US pop star Ariana Grande.
"It's like your own family just passed away, it's just so, so sad," 69-year-old Carmel McLaughlan told AFP, standing next to the sea of flowers filling the square.
As the nation mourned, Queen Elizabeth II visited children injured in the attack at a hospital in the northwestern English city.
"It's dreadful. Very wicked to target that sort of thing," she told Evie Mills, 14, and her parents.
Some 75 people are still being treated in hospital, including 23 in critical condition, medical officials said.
With investigators pushing ahead with the probe into the attack, British authorities were left "furious" by repeated leaks of material shared with their US counterparts that they said undermined the investigation.
In Brussels for a NATO summit on Thursday, Prime Minister Theresa May confronted Trump over the issue.
"She expressed the view that the intelligence sharing relationship we have with the US is hugely important and valuable, but that the information that we share should be kept secure," May's spokesman said.
Trump, who led NATO allies in paying respects to the victims, slammed the leaks as "deeply troubling".
"If appropriate, the culprit should be prosecuted to the fullest extent of the law," he said in a White House statement.
Images obtained by The New York Times newspaper showed a detonator Abedi was said to have carried in his left hand, shrapnel including nuts and screws and the shredded remains of a blue backpack.
Manchester police stopped passing information to Washington on their investigation but intelligence sharing later resumed, according to national anti-terrorism chief Mark Rowley.
"Having received fresh assurances, we are now working closely with our key partners around the world," he said.
University dropout Abedi, 22, grew up in a Libyan family that reportedly fled to Manchester to escape the now-fallen regime of Libyan dictator Moamer Kadhafi.
His father Ramadan and younger brother Hashem have been detained in Libya, with officials there saying the brother was aware of the planned attack.
They said both brothers belonged to the Islamic State group, while the father 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.
Libyan officials said Abedi's brother Hashem had been under surveillance for six weeks and that investigators had information he was planning "a terrorist attack" in Tripoli.
A relative told AFP that Abedi had travelled to Manchester from Libya four days before the bombing.
German police said he made a brief stopover at Duesseldorf airport, while a Turkish official said he had transited through Istanbul airport without saying where he was travelling from.
A source close to the family said Abedi wanted to avenge the murder in Manchester last year of a friend of Libyan descent, with his sister Jomana Abedi also telling The Wall Street Journal newspaper that he might have been driven by a desire for revenge.
"I think he saw children -- Muslim children -- dying everywhere, and wanted revenge. He saw the explosives America drops on children in Syria, and he wanted revenge," she said.
The bombing was the latest in a series of IS-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.
A British official confirmed Abedi had been on the intelligence radar before the massacre.
The MI5 domestic security service is managing around 500 active investigations, involving some 3,000 "subjects of interest", the senior government ministry source said.
"Abedi was one of a larger pool of former SOIs whose risk remained subject to review by MI5 and its partners," he said.
Police announced two new arrests on Thursday, bringing the total to eight people in custody in Britain.
Officers also briefly evacuated people from an area in Wigan, a town in Greater Manchester, as they searched a house in connection with the probe and a bomb disposal unit was deployed.
Britain's terror threat assessment has been hiked to "critical", the highest level, meaning an attack is considered imminent.
Armed troops have been sent to guard key sites, a rare sight in mainland Britain. Armed police were also deployed on trains for the first time ever.
Monday's attack was the deadliest in Britain since 2005 when four Islamist suicide bombers attacked London's transport system, killing 52 people.
The bombing occurred just over two weeks before a snap general election set for June 8 for which campaigning is set to resume in earnest on Friday after being suspended in the wake of the attack.