Demonstrators wrote messages like "I oppose genocide, I support Palestine Action" and chanted “shame on you” as arrests began.
The Metropolitan Police confirmed arrests were made for expressing support for a proscribed terror organisation.
Those being detained included a line of elderly protesters sitting silently on the edge of Parliament Square holding placards.
Palestine Action was proscribed in July after claiming responsibility for damaging two Voyager planes at RAF Brize Norton, making public support a criminal offence punishable by up to 14 years in prison.