British Prime Minister Theresa May has addressed the country, explaining what an increase to the security threat level from critical to severe means.
Ms May says the increased security level indicates another attack remains highly likely, and may be imminent.
"The work undertaken throughout the day has revealed that it is a possibility we cannot ignore, that there is a wider group of individuals linked to this attack."
The Prime Minister says additional resources and support may be made available to the police as they work to keep the country safe.
It could include members of the armed forces boosting security at key sites and military personnel might be deployed at public events.
At the heart of the latest attack, there were messages of hope and peace for the thousands who gathered in solidarity.
Eddy Newman, the Lord Mayor of Manchester, led the tributes.
"The people of Manchester will remember the victims forever and we will defy the terrorists by all our diverse communities working together cohesively and with mutual respect."
He was joined by the Bishop of Manchester, David Walker, who had a message for the world.
"We will stand together to say that this city is greater than the force that aligns itself against it. As we say that we are sending a signal not just to Manchester, but across the world, that you cannot defeat us because love in the end is always stronger than hate."
At the vigil were representatives of different religions who, one by one, condemned the bombing which ripped through a crowd leaving a concert by United States singer Ariana Grande.
Members of the city's Muslim, Christian, Jewish and Sikh communities said they wanted to show that Manchester, while shocked, would not be cowed.
Their mood was expressed by uplifting, inspiring words from local poet, Tony Walsh.
"This is a place that has been through some hard times,
Oppressions, recessions, depressions and dark times.
But we keep fighting back with Greater Manchester spirit, northern grit, northern wit and Greater Manchester lyrics.
And there's hard times again,
and there's hard times again,
there's hard times again in these streets of our city.
But we won't take defeat."
Flowers, candles and hand-written notes piled up as people came to pay their respects.
And then, in the warm evening sunshine in front of the Town Hall, the crowd erupted into an impromptu cheer.
Tributes will continue tomorrow, when Manchester United wear black armbands when they play in the Europa League Final.
There will also be a minute's silence for the victims.
United's executive vice-chairman, Ed Woodward, says the tragic event has affected the team.
"I think what happened last night really puts things into perspective. Success on the pitch really is nothing compared to the pain and suffering that's going on at home. That's really where our thoughts are at this point in time."
