A sword-wielding youth sparked panic when he led a charge across a station concourse minutes before a teenager was fatally attacked, a court has heard.
Commuter Warren Kelly said there was a general sense of panic when the male, followed by a group, openly brandished the weapon in Victoria station.
Sofyen Belamouadden, 15, died after being chased and stabbed during rush hour at the south London station.
Seven teenagers have denied murdering Sofyen in March last year.
It is alleged the Sofyen, from Acton, west London, was stabbed, punched, and kicked after being chased from the concourse down into the Tube.
In evidence read to the Old Bailey, Mr Kelly said: "I felt a general sense of panic in the area and saw members of the public jumping back and walking away from the immediate area."
He described seeing a youth, shouting at a group across the station concourse, with his right hand held in the air.
This was followed by a high-pitched female scream of panic and then "a large group of 10 to 15 males ran towards the shouting male - it seemed as fast as they could".
"I can only describe the male at the front - they were all sprinting," Mr Kelly said.
"In his right hand he was holding either a very large knife or a sword.
"This was very obvious - he was not trying to disguise the fact in any way. I was quite scared."
Obi Nwokeh, of Bermondsey, and Enoch Amoah and Samuel Rober, both from Camberwell, all aged 18, deny murder.
They have also pleaded not guilty to conspiracy to cause grievous bodily harm and violent disorder.
Four 17-year-old youths, one from Stockwell, one from Brixton, and two from Streatham, also deny the charges.
A total of 20 people are accused of Sofyen's murder, with further trials due to take place later in 2011.