The group of Jewish protesters massed in the main concourse of the busy station where they held a prayer service amid chants of “stop bombing Gaza” and “not in our name”.
Some wore cloaks saying “free Palestine” and “never again for anyone” as they called for an end to the conflict.
British Transport Police (BTP) officers were seen intervening before the group chanted in response, “let us pray”.
The group continued to chant until BTP officers eventually walked away.
A British Transport Police spokesperson said: "Just after 8am today, our officers attended a sit-in protest at St Pancras railway station. The group were reported to be peaceful and there were no reports of disorder. There was no impact on the station services and by 8.45am the group had moved on.
"Anyone wishing to make a report related to this incident can text 61016 or call 0800 40 50 40 with reference 116 of 2 November 2023."
Earlier on Thursday, a man was arrested daubing a building in central London with the words “free Palestine”.
Commander Kyle Gordon said there were 408 anti-Semitic incidents recorded in the capital in October, compared to 28 in the same period last year.
There were 174 Islamophobic offences compared to 65 in the same timeframe in 2022.
Amid escalating tensions at home and abroad, Sadiq Khan on Wednesday reiterated his call for a ceasefire, adding: “I unequivocally condemn Hamas and the actions of this terrorist organisation on October 7, the loss of more than 1,400 lives [in Israel]. They still have more than 200 people held hostage - they should be released immediately.
“I also think there’s been a lot of bloodshed, too much bloodshed, in Gaza. More than 9,000 lives lost, more than 3,000 children killed. I think we should have a de–escalation of violence, not an escalation of military violence, and I still do call for a ceasefire.”