More than a hundred people joined a protest at the city's Guildhall in Derry, in support of the people of Gaza and Palestine.
Some carried posters expressing support for Palestine Action.
The group has recently been categorised as a terrorist organisation by the UK authorities.
Organisers of the rally said people were deliberately flouting the law to highlight its contradictions.
"I think the idea that any law would stand against people holding posters rather than uphold international law to stop a genocide, is absolutely appalling," said Karen Moynihan, who is Ireland coordinator of the Global Movement to Gaza.
"I think everybody here is just looking at the absolute irony of it and just saying that we will stand with Palestine. We will do everything that we can.
"And if we're going to be demonized and arrested for holding a poster, I think our own national and international laws need to be looked at very strongly."
Ms Moynihan said people from Ireland would be amongst those taking a flotilla of small boats to Palestine at the end of this month in an attempt to deliver aid.
The protest in Derry was one of a series across the country in support of the people of Gaza and Palestine.
The biggest of them was at Shannon Airport, while there were also demonstrations in cities around the world.
