Pro-refugee protesters clashed with police outside King's Cross station, throwing smoke bombs at officers.
Activists staged a demonstration outside the station, where Eurostar trains come into London.
But dozens of protesters, many clad in black and with hoods up and their faces obscured with scarves, breached police barriers and stormed the station at around 6pm.
Scotland Yard said they tried to enter the Eurostar platforms at the station, but were stopped by officers.
A video posted on Twitter shows dozens of activists pushing their way through the police line and swarming into the station.
Photos show rows of British Transport Police in high-visibility yellow jackets guard the station as coloured smoke bombs go off around them.
Scotland Yard said: "Some protesters began to throw items at police, including smoke bombs.
"These protesters were ejected from the station and later dispersed from the area."
No arrests were made.
Joel Benjamin wrote on Twitter: "Utter carnage at Kings Cross station for migrants demo. Flares, Cops going in batons drawn."
British Transport Police said demonstrators arrived at the station at around 6pm and began a peaceful protest, adding: "Around this time a number of other individuals arrived at the station causing disorder and missiles, including smoke bombs, were thrown at police officers.
"This group were dispersed by police and officers remain at the station."