0433 GMT January 27, 2020
Celebrities were among the hundreds taking part in the World's Big Sleep Out in London, Edinburgh and Cardiff, BBC News reported.
Organizers of the World's Big Sleep Out were expecting more than 50,000 to take part globally, with other events in cities including New York, Brisbane and Dublin.
The initiative hopes to raise around £38 million ($50 million) for homelessness charities.
In London's Trafalgar Square, those camping out faced temperatures of about 10°C and heavy rain — conditions which supporters said rough sleepers face every day.
Dame Louise Casey, a former head of the UK government rough sleepers' unit and trustee of the Big Sleep Out, told the BBC she hoped the event would be "symbolic."
"It seems absolutely bloody crackers right now — the rain is so heavy — but we're doing it because basically the world has a homelessness problem, it has a displaced people problem, it has refugees," Dame Louise said.
"All of these people are here tonight walking in the shoes of people who are homeless, or people who are refugees, we're just experiencing something for one minute, we're experiencing something that people have to experience all year round.
"It is a privilege to be here this evening — wet and cold as it is."
The actress Dame Helen Mirren read bedtime stories to those camping in the square.
Meanwhile in Edinburgh, veteran actor Brian Cox spoke to crowds in West Princes Street Gardens and in New York, film star Will Smith delivered a speech.
The World's Big Sleep Out campaign was created by Josh Littlejohn, the cofounder of the Scottish charity and sandwich shop Social Bite.
Littlejohn said he wants "to send a message to the world's political leaders to enact compassionate policy and find solutions for homelessness locally and the global refugee crisis that affects us all."
The Office for National Statistics has said estimates for the number of people rough sleeping suggest numbers are increasing in England, Wales and Northern Ireland, but estimates based on homelessness applications suggest numbers are decreasing in Scotland.