The BBC reporter later appeared on television beside Hamas leader Ismail Haniyah and thanked everyone who had worked for his release.
He described his 114 days of captivity as "appalling."
"The last 16 weeks, of course, have been the very worst you can imagine of my life," Johnston said. "It was like being buried alive really, removed from the world, and, occasionally, terrifying. You were in the hands of people who were dangerous and unpredictable, and always frightening in that you didn't know when it might end."
British Prime Minister Gordon Brown expressed gratitude for Johnston's release during his first question time in the House of Commons today.
"The whole country will welcome the news that Alan Johnston, a fearless journalist whose voice was silenced for too long, is now free," Brown said. "And I want to thank all those who contributed to the diplomatic and other efforts to secure his freedom."
Johnston went missing on March 12 when his car was found abandoned. His captors later declared themselves to be from the Army of Islam. The little-known militant group said it would kill its captive if its demands for the release of Muslim prisoners in British custody were not met.
Johnston said he was kept in chains for one 24-hour period, but was not harmed physically until the last half-hour of his captivity, when his captors hit him "a bit."
His captors last week released a video showing Johnston wearing a vest lined with explosives, which he said would be detonated if efforts were made to free him by force.
Parents Express Relief
The correspondent's parents, Graham and Margaret Johnston, spoke to journalists outside their home in Scotland today.
"We are absolutely overjoyed," Graham Johnston said. "It has been 114 days of a living nightmare, and just to hear his voice -- he telephoned us, there was a lot of noise in the background, I think he was being [bumped] a lot, and all he said was, 'Hello, dad.' And I said, 'Hello, son. How are you? Are you all right?' He says, 'I'm 100 percent.' And then the phone [line] was cut. So, that's all we've had from him so far, but we've seen him on the [television], and it is just incredible. It has been a long 114 days."
And in London, the BBC's director of news, Helen Boaden, thanked people around the world for their support.
"We're incredibly relieved and overjoyed at Alan's safe release and also very, very grateful to all the people who have worked tirelessly behind the scenes to secure his safe release after 114 days," she said. "It's been a very, very difficult time and, of course, the thing that's kept us all going is the extraordinary support from people all around the world, people we've never met, who've made it clear they supported Alan."
Rallies worldwide called for Johnston's release, while an online petition was signed by some 200,000 people.
Johnston said he followed events on a radio he acquired about two weeks after his capture. At a press conference in Jerusalem today, Johnston expressed gratitude for the support shown him.
"I felt, at one point, you know, that all the journalists in the world were, kind of, coming to the rescue," he said. "At least, at least, they maybe couldn't rescue me, but they weren't going to let go. They weren't going to let the story die. And any kidnap victim will tell you that the thing they probably fear most of all is that the world is simply going to go on without them."
Hamas Facilitated Release
He said the seizure of power in Gaza by the Islamist Hamas movement and the group's subsequent pledge to improve security in the territory had facilitated his release.
"Hamas is a controversial organization with a lot of problems and so on in terms of relations with the outside world," Johnston said. "But I am pretty sure that if Hamas hadn't come in and stuck the heat on [the kidnappers] in a big way, I'd still be in that room."
Hamas gunmen overran Gaza last month, expelling their rivals from the Fatah faction of Palestinian Authority President Mahmud Abbas.
The Hamas military wing said it would actively work toward securing Johnston's release. But Hamas said today no deal had been struck with kidnappers to secure the release.
Hamas leader Ismail Haniyah expressed delight at Johnston's release.
"I said that serious, hard work, wisdom, and realism led to the closing of this case," Haniyah said. "All of this helped us to end this issue with honor for our Palestinian people. What is happening is an honorable image for the Palestinian people."
Hamas political leader Khalid Mish'al, said Johnston's release revealed the failings of the preceding Fatah administration.
Johnston has left the Gaza Strip for Israel en route to the United Kingdom.
(compiled from agency reports)