Former U.S. lawmaker and Green Party presidential candidate Cynthia McKinney, whose relief boat was seized by an Israeli naval ship Tuesday for the second time in a year, is demanding the immediate release of her and 20 other activists.

McKinney, a longtime supporter of Palestinians, said her Greek-flagged boat, the Spirit of Humanity, was carrying medical supplies, cement, olive trees and children's toys to Gaza when it was boarded by the Israeli navy.

"This is an outrageous violation of international law against us," said McKinney. "Our boat was not in Israeli waters and we were on a human rights mission to the Gaza Strip. President Obama just told Israel to let in humanitarian and reconstruction supplies, and that's exactly what we tried to do. We're asking the international community to demand our release so we can resume our journey."

The Israeli military issued a statement Tuesday saying that the boat had attempted to break a blockade of Gaza and was forced to sail to an Israeli port after ignoring a radio message to stay out of Gaza waters.

The statement said navy personnel boarded the freighter Arion without any shots being fired, and those on board were to be handed over to immigration authorities. Israeli Foreign Ministry spokesman Yigal Palmor said Israel was planning to free the crew and passengers. 

"Nobody wants to keep them here," he said. "They will be released as soon as they are checked."