A teenager has died after being found collapsed in a street in south-east London, with police saying he is believed to have taken nitrous oxide – also known as laughing gas – at a party beforehand. The alert was raised shortly after 11pm on Saturday when police and ambulance crews were called to a residential street in Bexley, to find an 18-year-old man in cardiac arrest.

The teenager, named on Sunday night as Ally Calvert from Sidcup, was taken to hospital in a critical condition, police said. He was pronounced dead at 1.30am on Sunday. The Metropolitan police said he was believed to have ingested nitrous oxide and alcohol at a party.

A Met statement said: “A postmortem will be scheduled in due course. At this stage the death is being treated as unexplained pending the findings of the postmortem. Inquiries continue.”

Two female friends leaving flowers at the entrance to a block of flats where Calvert collapsed said he had a brother and sister and planned to study to be an electrician. The teenagers said they had not seen the victim take nitrous oxide at the party, and that he had drunk just one beer. “He was the nicest person you could ever meet,” one said. “He was always happy and smiling.”

Local people said they had seen paramedics trying to resuscitate the teenager as he lay unconscious. One 22-year-old woman who asked not to be named said: “A policewoman was talking to two of his friends. I heard them say they had been at a party and were on their way home. It is a real tragedy. You see the empty canisters everywhere on the estate.”

Nitrous oxide, sometimes also known as “hippy crack” and used medically as a mild anaesthetic, is now the second most popular recreational drug in Britain after cannabis, with more than 400,000 16-to-24-year-olds reportedly taking it in the past year. Among the signs of its current ubiquity are the large numbers of discarded metal canisters seen at music festivals and other events. In 2014, organisers of the Glastonbury festival said they had collected two tonnes of the canisters.

Users generally inhale the gas from a balloon, which brings a brief period of euphoria. Hallucinations may occur with heavy use. While deaths connected to the drug are relatively rare – nine were documented between 2006 and 2012 – excessive use can bring oxygen deprivation, causing loss of blood pressure, fainting, and even heart attacks.

It is not illegal to possess or inhale nitrous oxide, and it is often sold openly, even though its sale is restricted under the Human Medicines Regulations Act of 2012. In May, the Home Office announced that sales of the drug would be restricted under a new clampdown on legal highs.