Athens, Greece (CNN) -- Greek police have arrested the captain of a U.S. ship whose passengers and crew had hoped to help lead a Gaza-bound flotilla aimed at challenging Israel's maritime blockade of the Palestinian territory.

John Klusmire, an American citizen and captain of The Audacity of Hope, was asked to report to a police station Saturday morning, said Jane Hirschmann, an American who is one of the flotilla organizers.

A spokesperson with the Greek Coast Guard, who was not named per custom, confirmed Klusmire's arrest. The reason given was that he had left the mainland without permission and put passengers in danger. He will appear before Greek prosecutors on Tuesday.

Hirschmann added that, in addition to disobeying a police order, Klusmire is being charged with disturbing sea traffic.

"This is intimidation," she said. "They're making an example of our captain to stop other boats in the flotilla from trying to sail."

His vessel -- which carried 36 American passengers, four crew members and several journalists, including CNN reporter Phil Black -- was intercepted Friday by a Greek Coast Guard patrol vessel roughly 10 minutes after it had departed Perama.

The Greek authorities were in a standoff with the crew for about three hours, during which activists sang, chanted and waved signs in support of their mission. The ordeal took a turn, however, when an inflatable boat carrying an armed Greek military commando team told the activists to follow them back to port or face being boarded.

Eventually, the ship returned to Greece, where it is now tied up at a Greek naval facility.

Four other crew members have been ordered by Greek authorities to stay aboard the ship, Hirschmann said. Many other activist passengers have remained on the vessel as well, in a show of solidarity.

The Audacity of Hope is one of 10 ships that organizers hope to unite to form a flotilla heading toward the Middle East. The trip is partially meant to commemorate a May 2010 incident in which Israeli troops boarded Mavi Marmara, a Turkish ship filled with humanitarian aid and 700 activists from various countries. Nine people died in subsequent clashes with Israeli Navy commandos.

The Greek government said Friday that it won't allow any of the nine other boats in the flotilla to sail to Gaza because their mission is too dangerous. Flotilla members accuse the Greek government of buckling to pressure from Israel's government.

For their part, the Israelis have said they are mounting a diplomatic offensive to try to stop the flotilla from setting sail at all.

On Tuesday, the Israeli government said flotilla participants had threatened to kill Israeli military personnel should their boats be boarded. They alleged that the participants were stockpiling sacks of sulfuric acid on boats to be used in the event of any attacks on Israeli commandos.

Flotilla organizer Medea Benjamin dismissed the allegations as ludicrous, saying: "They see this nonviolent, rag-tag group of ships as such a threat they're using their entire propaganda apparatus, and their diplomatic and economic clout to try to stop 300 peace activists. It's pathetic."

The Middle East Quartet -- composed of officials from the United States, United Nations, European Union and Russia -- on Saturday issued a joint statement asking "all governments concerned to use their influence to discourage additional flotillas, which risk the safety of their participants and carry the potential for escalation."

The Quartet added in its statement that it "strongly urges all those wishing to deliver goods to the people of Gaza to do so through established channels, so that their cargo can be inspected and transferred via established land crossings." Doing so, the group contends, is the best way to both help Palestinians and address Israel's "legitimate security concerns."

Journalist Elinda Labropoulou contributed to this report.