The decision to proscribe the group came into effect at midnighht on Saturday.
More than 20 people have been arrested at a protest following the decision to proscribe Palestine Action as a terrorist organisation.
A last-minute legal challenge at the Court of Appeal to delay it failed.
Several protestors met today in London’s Parliament Square in relation to the decision which falls under the Terrorism Act 2000.
The Metropolitan Police said: “The group is now proscribed and expressing support for them is a criminal offence.
“Arrests are being made.”
Home Secretary Yvette Cooper made the deciosn to proscribe the group which was voted in favour overwhelmingly by MPs.
The decision now means that the organisation now joins a list with the likes of al Qaeda, ISIS and Hezbollah.
Membership of, or support for, the direct action group is a criminal offence punishable by up to 14 years in prison, reports Sky News.
Wearing memorabilia, such as t-shirts or badges with the group’s name, can land you a maximum six-month sentence.
Protestors turned out at Parliament Square in large numbers, holding placards saying: “I oppose genocide. I support Palestine Action”.
Police officers who carried out arrests were met with shouts of “Met Police, you are puppets of the Zionist state”, and “leave them alone”.
According to reports, other people were heard shouting “British police off our streets” and “From the river to the sea, Palestine will be free”.
Included in those arrested was 83-year-old Reverend Sue Parfitt.
Speaking to Sky Leslie Tate, 76, a Green councillor from Hertfordshire, said: “Palestine Action are not a violent organisation, and the proscription is wrong.
“This protest is necessary to defend our democracy, and this is the creeping edge of totalitarianism, frankly.”
