An illegal rave alerted police to one of the largest squats in Brighton and Hove.
Scores of revellers, many believed to be on drugs, were spotted leaving the derelict property in North Street, Brighton, on Sunday.
Police yesterday raided the building, evicting eight squatters and a two-year-old boy, shortly after noon.
Officers also arrested a suspected illegal immigrant, a man in his 20s from Colombia. They searched the property with a sniffer dog trained to find drugs.
Squatters had converted up to 25 rooms on the first, second and third floors into a make-shift home, spanning above three businesses in the busy shopping street.
A hoard of sound equipment was found in the basement, along with hundreds of discarded drinks cans and drug paraphernalia left behind by ravers.
Inspector Nev Kemp, of Sussex Police, said: "There was an illegal rave held at the property and officers noticed people leaving on Sunday who appeared to be under the influence of drugs.
"As a result we conducted a search of the building and found eight adults and one, two-year old child.
"This was one of the largest properties we have ever had to search in the city and they can very quickly become associated with crime so we want to be positive in dealing with them."
Squatters, including the mother and her two-year-old son, were believed to have been living there for three weeks.
One room was filled with children's toys and was being used as a nursery.
The group were sleeping in empty rooms on the top floor and had access to running water and electricity.
Police also confiscated anti-war literature including posters and CDs.
An inspection ruled the site was a fire risk before squatters were ordered to leave.
Police then boarded up the windows and doors.
The mother and child were referred to Brighton and Hove City Council social services to find emergency accommodation.
A man, aged 34, who did not want to be named, said he had organised the illegal rave in aid of charity. He said party-goers donated £600 for refugees in Africa. He said: "There is a group of us who raise money doing parties and there was an awful lot of good happening at the building."
The discovery of the illegal squat comes a week after police raided another abandoned building in a nearby alleyway off Duke Street.
The derelict nightclub and cafe behind Burger King was being used by up to 40 people.
Police raided the building on Monday last week, evicting eight squatters.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article