Counter terrorism detectives made the arrest at around 7.30am this morning (January 31) in central London.
The alleged terrorism offence took place at a pro-Palestine protest in Camden on October 19 last year.
The woman was arrested on suspicion of inviting support for Hamas, a proscribed terrorist organisation in the UK.
She is currently in police custody and officers have completed a search of an address in central London.
A police spokesperson said: “The arrest shows the Met’s continued commitment to pursuing to the fullest extent of the law allegations of support for a terrorist group.”