THE CANADIAN PRESS -- OTTAWA - The Greek coast guard has crippled a Canadian ship bound for Gaza, forcing it back to shore and ramming it against a cement pier, activists said Monday.

Activists on board say the ship appears to be sinking and they're calling on the Harper government to intervene.

The Tahrir, bearing at least 30 Canadians, left a Greek port early Monday evening local time and was boarded 15 minutes later by armed officers from the Greek coast guard.

Within an hour, the ship was back at another Greek port, where supporters of its mission were gathering.

"They arrived back to land into a location ... where the coast guard forced them to go to. Not the original dock," Ehab Lotayef, a Montreal-based spokesman for the ship, tells The Canadian Press.

"They slammed it into the concrete, into the pier-side. At the beginning it wasn't obvious, but now there is big probability the boat is sinking. It got damaged in that slam. It's taking water, leaking diesel."

Lotayef said efforts were underway to get passengers off the boat.

"We are holding the Greeks responsible for all that — totally unjustified action by the Greek government, arm-twisted by the Israelis," he said. "It is a setback but it's not the end of the road; the efforts will continue worldwide until this blockade ends."

Two Canadian activists on board the ship have provided The Canadian Press with a dramatic description of the ship's boarding.

"We're slowly heading back. There's a new boat that's pulled up beside ... must be another coast guard vessel," Dylan Penner said in a mobile telephone interview from the Greek-occupied vessel.

Penner said eight to 10 armed coast guard officers boarded the ship and soldiers were in command of its bridge. They were armed with handguns and assault rifles.

The ship had left the Greek port of Agios Nikolaos. Days earlier, a Greek coast guard vessel took up a berth next to the Tahrir after its activists declared their intent to set sail after various delays.

Within 15 minutes, two armed coast guard officers managed to reach the Tahrir, and began the boarding.

No one on board was believed to be injured, and no shots were fired, but the Greek coast guard blasted a water cannon at the ship as it left port, said Penner.

A coast guard officer with his pistol drawn stepped over Penner as he sat in peaceful process on the ship's deck.

Penner called on Ottawa to denounce what they view as an unlawful act by the Greek government.

"This is a peaceful international mission to break Israel's illegal blockade of Gaza which, it is now crystal clear, has been extended to the ports of Greece," said Penner.

"I wouldn't say it's a surprise but it's certainly shameful. We have every right to bring the aid we are attempting to bring to Gaza to break the siege," he added.

"We have every right to uphold international law and in fact we have a responsibility to do so."

The Tahrir was one of several vessels aiming to breach Israel's sea blockade of the Gaza Strip in an attempt to deliver the activists' aid.

"The Greek military and the Greek government prefers to send soldiers against a boat full of people — parents, grandparents . . . Jews, Christians, Muslims . . . people from all over Quebec and Canada but also Danes, Australians, Belgians who raised money to send tens of thousands of dollars of medical equipment to Gaza," said Stephan Corriveau, another Canadian on the ship.

The Harper government has criticized the flotilla.

"Our position has not changed since Minister Baird issued a statement on May 28," said Chris Day, a spokesman for Foreign Affairs Minister John Baird.

"We will provide consular assistance as required."

Baird had strongly urged those wishing to deliver humanitarian goods to Gaza to do so through established channels such as the International Committee of the Red Cross/Red Crescent.

"Unauthorized efforts to deliver aid are provocative and, ultimately, unhelpful to the people of Gaza," Baird said in his May statement.

"Canada recognizes Israel’s legitimate security concerns and its right to protect itself and its residents from attacks by Hamas and other terrorist groups, including by preventing the smuggling of weapons."

Flotilla organizers pledged Sunday to continue with their plans to breach Israel's sea blockade of the territory despite a Greek government ban on their vessels leaving the country's ports.

The campaign experienced a major setback when Greece announced its restrictions Friday. Authorities arrested the captain of a boat carrying U.S. activists that tried to leave Greece without permission.

The Greek Merchant Marine Ministry said the coast guard blocked the boat shortly after it set sail without permission from the port of Agios Nikolaos in northeastern Crete.

The ministry said an operation was underway to return it to port.

Greece banned all Gaza flotilla boats from leaving port, citing security concerns. Last year, nine activists were killed after Israeli commandos boarded a Turkish boat taking part in a similar flotilla.

The Greek foreign ministry has offered to deliver the humanitarian aid the activists want to take to Gaza itself.

Greek foreign ministry spokesman Grigoris Delavekouras defended his government's action.

"Greece is taking a responsible stance and dealing with a particular situation. And this situation says that there is an immediate danger to human life by participating in this attempt," Delavekouras said.

"This is something that arises from experience. I think we all remember the tragic events we had last year. The region doesn't need this at the moment."

Israel says it imposed the blockade in 2007 to stop weapons reaching Hamas, the Islamic militant group that rules Gaza.

Israel eased its land blockade after an international uproar over last year's raid on the Turkish boat.

Asked why he chose to set said with the Tahrir, even though it was well know that Greece would intervene, Corriveau replied:

"I didn't come here to spend my vacation in Greece. I came here because I had a mission and I'm not the only one."

Lotayef said supporters of the cause raised about $350,000 in Canada from private individuals and civil society groups to buy the ship from its Greek owners, and to modify it to be part of the flotilla.

Its sails under the colours of the Comoros Islands, a flag of convenience that is not uncommon, he added.

"It's bought by Canadian money. Its a project that was initiated in Canada. All the work behind it is Canadian civil society support," said Lotayef.

"It was bought for that purpose — to be a Canadian boat that would go and break the blockade of Gaza."

The vessel was named after Tahrir Square in Cairo, the centre of the protests that eventually forced Egyptian dictator Hosni Mubarak out of power earlier this year.