ATHENS, Greece - Organizers of a Gaza-bound flotilla said Sunday they have not abandoned 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, and authorities arrested the captain of a boat carrying American activists that tried to leave Greece without permission.

However, coordinators of the flotilla were trying to maintain momentum with small protests in Athens in the face of increasing calls for them to scrap their campaign. On Saturday, the Middle East Quartet of Mideast mediators - the US, UN, EU and Russia - urged governments to discourage Gaza-bound flotillas that could escalate tension in the region.

Several protesters from the American vessel briefly protested outside the US Embassy before leaving. They had originally planned to stay there overnight. They want Washington to pressure Greece to release their American captain, John Klusmire, and allow them to depart for the Gaza Strip.

The US Embassy provided standard consular services to the American citizens in the flotilla "before and after" the interception of their vessel, spokesman Stuart Smith said.

The Obama administration had warned American activists against participating in the flotilla, saying they might be violating US law because Gaza is run by the militant Hamas group, a US-designated foreign terrorist organization.

Greek activist Dimitris Plionis said there would be "some action" at the beginning of the week, but he did not specify what the pro-Palestinian activists were planning to do, presumably because Greek authorities might try to thwart their efforts.

"The ban is there and we have already said that we are still considering to sail," Plionis said. "This story is not finished."

Without elaborating, he noted that "ships are free to go to other locations" besides Gaza.

The comment raised the possibility that organizers have debated whether their vessels could declare they are bound for another destination, and then turn toward Gaza once they are in international waters. The flotilla has planned to carry medicine, construction equipment and other aid to the coastal strip.

Pro-Palestinian groups said hundreds of activists from around the world intend to fly to Israel's airport Friday to protest the country's naval blockade of the Gaza Strip and other policies. By staging large demonstrations at the airport, Israeli media say the activists plan on disrupting flights. Media reported that already tight security at the airport will be beefed up. Police refused to comment on operational preparations.

The groups that called Sunday for the protest are based in Europe and the US. They include the International Solidarity Movement, a pro-Palestinian group that often sends activists to the West Bank and Gaza.

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

Nine activists on a Turkish boat were killed last year in an Israeli raid on a similar flotilla, and Israel eased its land blockade after an international uproar over the incident. But Israel has pledged to thwart any attempt to reach Gaza by sea, and that aid deliveries can occur through its own established channels.

Activists reject that option, saying Israeli restrictions on the Palestinian territory of 1.5 million amount to a human rights violation. Egypt recently lifted its own blockade of Gaza at the Rafah crossing, though cross-border traffic is still slow.

In an acknowledgment of the challenges facing the flotilla, a spokesman for a Turkish group that dispatched the boat that was raided by Israel in 2010 said it was "not impossible" that the vessels would head to Gaza. Huseyin Oruc of IHH, an Islamic aid group that earlier pulled its boat out of this year's flotilla, spoke to The Associated Press in the Greek capital.

IHH has remained involved in flotilla planning. Israel accuses the group of having terrorist links, an allegation its directors strongly deny. IHH, which operates aid projects in many nations, is not on a US list of terror organizations.

Planners had originally talked of sending 15 vessels with up to 1,500 people in the flotilla this year, though that number has dwindled to several hundred activists and fewer than 10 boats.

In a statement, the Greek Foreign Ministry said it had banned the flotilla from sailing out of concern for "the protection and safety of human life," while noting the need to lift the Gaza blockade and improve humanitarian conditions there.

"Greece reiterates its willingness and proposes to undertake the task of transporting the humanitarian aid, with Greek vessels or other appropriate means, through the existing channels" in line with a UN request, the ministry said.

Members of the flotilla have accused Greece of succumbing to Israeli pressure to block their plans, which had been held up by administrative delays and legal entanglements prior to the ban on sailing.

Greece, keen to boost international investment and ease its financial crisis, has maintained close relations with Arab countries, but in recent years has forged closer military and economic ties with Israel. The shift came amid a deterioration in the Jewish state's ties with Israel. The shift came amid a deterioration in the Jewish state's ties with Turkey, Greece's traditional regional rival and a fierce critic of the three-week Gaza offensive launched in late 2008 by the Israeli military.

Prime Minister Benjamin Netanyahu of Israel singled out Greece last week when he praised a group of foreign countries for helping to block the flotilla. In a speech, he thanked "my friend, the Greek Prime Minister George Papandreou."

On Sunday, Papandreou's office said he had communicated with Palestinian President Mahmoud Abbas, who expressed support for the Greek government's proposal to transport the activists' aid to Gaza, which would require Israeli cooperation.

Abbas' Fatah Party and Hamas have been trying to end a four-year-rift that left the Palestinians with two rival governments in the West Bank and Gaza.

Robert Naiman, an American activist, dismissed the Greek offer as an attempt by Greece to deflect criticism over its ban on the flotilla departure.