A crackdown on people carrying knives has been launched by British Transport Police, with officers using an airport-style metal detector.
The portable scanner will be moved around London
Wembley Central station was chosen for the first stage of the campaign, with a portable scanner backed up by officers with stop-and-search powers.
Two people were arrested for carrying knives, police said.
The scanner is being used in Harlesden later on Tuesday, and in other areas of the city during a six-week trial.
A 17-year-old from Leytonstone was arrested for possession of an offensive weapon, while a 25-year-old County Durham man was arrested on a warrant for criminal damage.
The scheme may be rolled out nationally, a British Transport Police spokesman said.
He said: "We think it sends a message. Obviously, the officers at the station will be looking for people who turn back when they see the police line, and we will be stopping them.
"Certainly, we've had groups of youths turned away from the station."
He said the use of the detector was partly a reaction to the fatal stabbing of lawyer Thomas ap Rhys Price three weeks ago near Kensal Green station. Two teenagers have been charged with robbing and murdering him.
The spokesman said use of the detectors had been planned for some time.
He said the public had welcomed the move, and said most people's journeys had not been disrupted because police were only targeting young men aged between 16 and 30.
Some youths interviewed by BBC London were unhappy. One Harlesden resident, Elias Akayeampong, said he had searched despite being a victim of a knife crime himself.
"Everyone carries a knife... it's a fact. It's a protection. People do it to protect themselves," he said, adding he did think it was right to carry knives.
Brent South MP Dan Butler said the area did not feel safe at times and the stations should be staffed constantly.