Israel’s military said in a statement that the vessel was boarded without incident and that “no violence of any kind was used by neither the passengers onboard nor the Israel naval forces.”

Image A small group of left-wing Israelis waited on the beach north of Ashdod on Tuesday for the catamaran trying to break the blockade to be escorted into the harbor. Credit... Jim Hollander/European Pressphoto Agency

But an army spokesman’s account was countered at least partially by Smadar Ben-Natan, a lawyer representing the group. She said that two brothers on board, one a former Israeli air force pilot and high-profile peace activist, were mistreated by the forces during the operation.

Ms. Ben-Natan said Yonatan Shapira, 38, the former pilot, was struck by a stun gun and that both he and his brother Itamar, 30, a former combat soldier, were “treated brutally and handcuffed and taken off the boat while the others were treated better.” She added that, over all, the seizure was peaceful.

The boat’s voyage from Cyprus was another attempt to thwart the blockade after an Israeli assault on a Turkish Gaza-bound aid boat on May 31 in which Israeli commando forces opened fire, killing nine Turkish Islamic activists on board and setting off an international dispute.

Israel maintains that its forces operated in self-defense after they said they came under attack by passengers. Last week a United Nations Human Rights Council investigation concluded that Israel violated international law in the raid. Israel dismissed the report as biased, and on Tuesday the United States said that its findings were “unbalanced” but that it had heard no factual criticism of the report.