Israel on Monday deported the Swedish activist Greta Thunberg and about 170 other participants in a flotilla that tried to deliver aid by sea to Gaza but was intercepted by the Israeli authorities, the foreign ministry said.

The widely publicized mission, involving dozens of boats and hundreds of activists, aimed to breach an Israeli blockade of Gaza, where hunger is widespread and a U.N.-backed panel of experts has declared that famine afflicts hundreds of thousands of people.

Israel, which has limited deliveries to the territory for almost two decades, has imposed stringent restrictions on the entry of food and other aid since the war there began two years ago. For more than two months earlier this year, it prevented any food from being brought in.

Flotilla participants say that Israeli forces illegally intercepted their boats last week in violation of maritime law and international humanitarian law. Israel says the activists violated a legal blockade.

Some who were arrested and then released over the weekend reported that they had been mistreated in Israeli custody. Israel’s foreign ministry denied the accusation in statements on Monday and over the weekend. “All the legal rights of the participants in this P.R. stunt were and will continue to be fully upheld,” the ministry said, accusing activists of spreading “fake news.”

There had been reports from activists and others over the weekend that Ms. Thunberg was mistreated. Ms. Thunberg confirmed those reports on Monday. She was among the participants deported to Greece and addressed crowds waiting for the activists upon their arrival in Athens.

“I could talk for a very, very long time about our mistreatment and abuses in our imprisonment, trust me, but that is not the story,” she said, according to a video of her remarks posted by flotilla organizers.

Ms. Thunberg turned discussion to the plight of Palestinians. She accused Israel of committing genocide and attempting to erase a population before the world’s eyes. A United Nations commission investigating the war in Gaza last month concluded that Israel was committing genocide, allegations that Israel has repeatedly rejected.

Those deported on Monday were sent to Greece and Slovakia, the Israeli foreign ministry said. “The deportees are citizens of Greece, Italy, France, Ireland, Sweden, Poland, Germany, Bulgaria, Lithuania, Austria, Luxembourg, Finland, Denmark, Slovakia, Switzerland, Norway, the U.K., Serbia and the United States,” it said.

On Sunday, the ministry said that Israel had sent to Spain 29 flotilla participants from Spain, Portugal and the Netherlands. On Saturday, 137 people from the United States, Italy, the United Kingdom, Jordan, Kuwait, Libya, Algeria, Mauritania, Malaysia, Bahrain, Morocco, Switzerland, Tunisia, and Turkey were deported to Turkey, it said.

A livestream posted by the flotilla organizers showed crowds of what appeared to be hundreds of people waiting for the activists at the airport in Athens. “Free, free Palestine,” they chanted. The flotilla participants were greeted warmly by the crowds.

Miriam Azem, a spokeswoman for Adalah, a legal organization for Arab minority rights in Israel that is representing the flotilla participants, said in an interview on Sunday night that lawyers who met with activists were told they had been kept on their knees with their hands bound for many hours. They were held in overcrowded cells and denied adequate drinking water, she said, and some said they had also been denied food and critical medications in the initial days and were deliberately deprived of sleep.

Some flotilla participants were on a hunger strike while in custody, Ms. Azem and the group’s organizers have said.

Ms. Azem said that there had been numerous due process violations, with lawyers for the group not notified that hearings were taking place and activists denied access to counsel. She said that Israel’s process was “entirely illegal,” with activists forcibly taken to the country, then treated as having entered illegally and detained in a security prison. She spoke of “a cycle that is meant to intimidate and to deter.”

On Monday, Ms. Azem said that about 140 flotilla participants were still being detained, but said that the organization could confirm the number and that it was receiving little information from the Israeli authorities.

Relatives of some of the American participants on the flotilla — there were about 20 — met in Capitol Hill on Monday to press lawmakers for their release.

Representative Ro Khanna, Democrat of California, was drafting a letter to be sent on Monday afternoon to Secretary of State Marco Rubio inquiring about four Californians aboard the flotilla who were detained, along with other Americans.

The participants were on a “nonviolent mission to deliver desperately needed humanitarian aid to Palestinians in Gaza,” Mr. Khanna wrote, according to a draft of the letter shared with The New York Times. He called for more aid to enter Gaza in addition to protection of flotilla participants.

“The U.S. has an obligation to protect its citizens abroad and must act immediately.” Mr. Khanna wrote.

Niki Kitsantonis and Johnatan Reiss contributed reporting.