Five British Greenpeace activists arrived home in defiant mood on Friday after Russia granted them an amnesty to halt their prosecution for protesting against oil drilling in the Arctic.

Precisely 100 days after they were arrested on a Greenpeace ship, they flew from Saint Petersburg to Paris and then took a Eurostar train to London.

Anthony Perrett, Phil Ball, Iain Rogers, Alex Harris and filmmaker Kieron Bryan smiled as they posed for a scrum of photographers before emotional family reunions in the arrivals hall of St Pancras station.

Perrett said it was "good to be back" and he was "looking forward to spending some time in the woods" in his native Wales.

He said conditions in the prison in the Arctic Circle city of Murmansk, where the group of 30 activists were first held, were "a real challenge" and admitted he had experienced "quite a few dark moments".

Perrett said there had been deep snow and they were held in their cells for 23 hours a day, sharing a toilet between three people.

They were later moved to a more comfortable prison in Saint Petersburg before being released from custody after two months. They were allowed to leave Russia after the Kremlin-backed amnesty was issued.

Asked if the protest had achieved Greenpeace's aims, Perret said: "Of course it has been worth it. Our mission is to save the Arctic and stop oil exploration and we've never enjoyed quite so much media."

He said it was "completely preposterous" that the group had initially been charged with piracy, which carries a prison sentence of up to 15 years under Russian law.

The Greenpeace activists had been on board the Dutch-flagged ship Arctic Sunrise, targeting an offshore oil rig owned by the Russian energy giant Gazprom when they were seized in September by Russian security forces who winched down from a helicopter.

Seven of the so-called Arctic 30 charged in the probe have now left Russia after Dmitri Litvinov, a Swedish-American, left Saint Petersburg for Helsinki on Thursday.

Alexandre Paul of Canada flew out of Russia on Friday with the five Britons.

'I won't stop Arctic protests now'

Alex Harris, the Greenpeace communications officer on the ship, said she thought the Russian government had granted the amnesty to avoid global criticism with just weeks to go until the start of the 2014 Sochi Winter Olympics.

"I think it was the easy way out for Russia, to get rid of us before the Olympics began and before there's a big PR pressure from Greenpeace and the rest of the world," she told journalists.

The activist said she too had experienced appalling conditions in the Murmansk prison.

"The conditions are really bad, it's cold, the food is unedible -- it's not good," she said. "I had a leach on my toothbrush at one point.

"There was no physical violence towards me but it was torture -- we spent two months in a Russian jail cell and 100 days detained for a crime we didn't commit.

But she insisted she would carry on "peaceful campaigning" against Arctic oil exploration.

"I have gone through a lot for this campaign and I am not going to stop now."

The arrest of the Arctic 30 -- who hail from 18 different countries -- risked becoming another bone of contention in increasingly tense relations between Russia and the West.

Russia's Federal Migration Service said all the 26 foreigners will have been given exit visas by the end of Friday. The other four activists are Russians.

Peter Willcox, the veteran captain of the Arctic Sunrise, was also expected to leave Russia on Friday. He was captain of Greenpeace's Rainbow Warrior ship when it was bombed by the French secret service in port in New Zealand in 1985.

Dutch activist Faiza Oulahsen, 26, told AFP before leaving Russia on Friday she also had "no regrets" over the protest and it had made her "even more dedicated" to save the Arctic.

But even as the Greenpeace activists left Russia, Gazprom announced on Friday it had begun oil production at the Prirazlomnaya oil rig that was the focus of their actions.

Two jailed members of the Russian punk band Pussy Riot, Nadezhda Tolokonnikova and Maria Alyokhina, were freed on Monday after benefitting from the same amnesty.