Mordechai Vanunu has been freed after an 18-year jail sentence for leaking Israel's atomic weapons secrets.
Vanunu said 18 years in jail had not broken his spirit
To jeers and cheers from a crowd at the southern Israeli prison, he said he was proud of his actions but had suffered "very cruel and barbaric treatment".
He said: "Israel doesn't need nuclear arms. My message today to the world is: Open Dimona reactor for inspections."
A Christian convert, Mr Vanunu's first stop was St George's Anglican cathedral in Jerusalem where he went to pray.
Mr Vanunu had emerged from Shikma prison in Askhelon shortly after 1200 (0900GMT) looking happy and waving to hundreds of Israeli and foreign supporters gathered outside the gates.
A smaller counter-demonstration by Israelis shouted abuse as he made Victory-V signs with both hands.
Answering questions in English from reporters, Mr Vanunu said he was "not totally free" because of the restrictions Israel had placed on him, including a ban on travel outside the country.
But he said he wanted to go to the United States, get married and start a family.
He said he had been treated harshly because of his conversion to Christianity and he no longer had any secrets to publish.
Despite his protestations, the authorities say Mr Vanunu still possesses information that could jeopardise Israel's security.
"He is a man sentenced to prison for treason and he has repeatedly said he will go back to his old behaviour... [Israel] has to take precautions to prevent that from happening," said Foreign Ministry spokesman Jonathan Peled.
Mr Vanunu is not allowed to have a passport, is forbidden to approach ports and airports, and has been told not to talk to foreigners without permission.
Mr Peled told the BBC that he was allowed to talk to the international media, as long as he did not talk about his work at the Dimona plant.
"We have to strike the correct balance between safeguarding his personal liberty and looking after Israel's national security interests," Mr Peled said.
On the basis of the information he gave to the UK's Sunday Times newspaper in 1986, analysts concluded Israel had scores of nuclear warheads.
Israel maintains a policy of "strategic ambiguity" about its supposed nuclear arsenal and it has refused to sign the Nuclear Non-Proliferation Treaty which would open Dimona up to international scrutiny.
Supporters had gathered outside the jail, waving banners and calling Vanunu a "hero of peace". But many in Israel see him as a traitor who has endangered the country.
Israel said it could have placed much tougher post-release restrictions on Mr Vanunu - and the length of time the current regime will remain in force depends on his behaviour.