Celebrities were among the hundreds taking part in the World's Big Sleep Out in London, Edinburgh and Cardiff on Saturday night, the BBC said in a report.
Organisers 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 $50 million for homelessness charities.
In London's Trafalgar Square, those camping out faced temperatures of about 10 degrees Celsius and heavy rain conditions which supporters said rough sleepers face every day.
Louise Casey, a former head of the government rough sleepers' unit and trustee of the Big Sleep Out, told the BBC that she hoped the event would be "symbolic".
British actress Helen Mirren read bedtime stories to those camping in the square while the band Travis played a set.
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 UK's 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 were decreasing in Scotland.
The World's Big Sleep Out campaign was created by Josh Littlejohn, the co-founder of the Scottish charity and sandwich shop Social Bite. The charity has hosted visits from a number of celebrities, including Hollywood actors George Clooney and Leonardo DiCaprio.
( With inputs from IANS )