Green Party leader Natalie Bennett has said it should not be illegal to join terrorist organisations.
Ms Bennett said people should not be punished "for what they think or what they believe".
In an interview with the BBC's Sunday Politics, she also defended the Greens' plans for a "basic citizen's income" of £72 a week, costing up to £280bn.
She said half of this "gross" cost would be covered by savings made by scrapping the current benefits system.
The government currently maintains a list of "proscribed organisations" which are banned by law. It is illegal to belong, or profess to belong, to an organisation on the list.
According to Green Party policy: "It should not be a crime simply to belong to an organisation or have sympathy with its aims, though it should be a crime to aid and abet criminal acts or deliberately fund such acts."
Asked whether this meant people would be allowed to be a member of al-Qaeda or Islamic State, Ms Bennett said: "Exactly. What we want to do is make sure we are not punishing people for what they think or what they believe.
"Obviously actions of inciting violence, supporting violence, those are absolutely unacceptable, illegal and should be pursued to the full extent of the law."
She added: "What we are talking about is a principle that you shouldn't be punished for what you think. And we need to balance, we do not protect freedom by destroying it."
The Green Party, which has been invited to join the live TV election debates under revised plans put forward last week, would also scrap the UK's nuclear deterrent, scale back the armed forces and reduce border controls.
Asked about funding the citizen's income - which would be paid to all people in the UK regardless of wealth or need - she added: "All of this will be fully spelled out in our fully-costed manifesto released in March that will set out exactly how we are going to make all of this add up."