An illegal rave in Brentford attracted up to 900 revellers, who police said risked their lives crossing train tracks to reach the venue.
Ravers at the illicit Valentine's Day gathering, in disused offices near Kew Bridge station, were also getting high on laughing gas, according to a sergeant whose officers helped police the event.
Sergeant Dave Turtle, of Turnham Green ward, said officers had been called to the rave in offices off Capital Interchange Way, on Saturday (February 14) night, by residents and by the rail operator, which was concerned about trespassers on the tracks.
"There were up to 900 people at one point and they were going on the railway tracks to get to the site," he told getwestlondon. "It appeared to be far enough from people's homes not to be a noise nuisance but there were health and safety issues.
"As well as trespassing on the tracks, people were using gas canisters like those seen at Notting Hill Carnival to get high.
"We monitored the event and the organisers turned off the sound system some time between midnight and 1am. Due to the large numbers involved, it took a long time for the crowds to disperse.
"We had about 10 officers there at one stage and we had to call the fire brigade to open a barrier so we could get vehicles through if necessary."
He added that nobody had been arrested.
Laughing gas, or nitrous oxide, is a legal high, which is reported to be widely used among young people.
Councils have warned of the dangers of taking the substance, which they say can lead to fainting, heart attacks and poisoning of the central nervous system.