An Italian activist found dead here early Friday was strangled with a plastic cord, apparently by abductors from a radical Islamic organization inspired by Al Qaeda that said it had kidnapped him a day earlier, according to a physician who performed an autopsy.

Vittorio Arrigoni taking part in March in a protest against gainst an Israeli decision to tighten the border area in Khan Yunis, south of the Gaza strip.

Police officers from Hamas, the group that rules Gaza, found the body of the pro-Palestinian activist, Vittorio Arrigoni, 36, in a house in Gaza City. The house was empty of furniture, other than a mattress on which the body was lying, according to witnesses.

The group that claimed the abduction, known as Tawhid and Jihad, had threatened to execute the Italian unless Hamas released its imprisoned leader, Hisham Saidani, who was arrested by Hamas forces in March. In a video released in the group’s name on Thursday, the kidnappers set a 30-hour deadline that was to expire at 5 p.m. on Friday.

But the Palestinian doctor who performed an autopsy said the hostage seemed to have been killed at least 24 hours earlier.

Tawhid and Jihad issued a denial of responsibility on Friday, but there was no way of verifying that claim, nor the earlier claim of responsibility on the video.

The video, similar to those released by extremists in Iraq and Afghanistan, showed Mr. Arrigoni blindfolded and being held roughly by the hair. Only the outstretched arm of the hidden captor was visible. Mr. Arrigoni was a familiar face in Gaza, where he was known as Victor.

The doctor who performed the autopsy said that Mr. Arrigoni’s hands had been tied and that there were signs of beating with a sharp object above and behind his ear. The doctor spoke on condition of anonymity because the official results of the autopsy had not yet been released.

The Hamas authorities released a brief statement at dawn on Friday, confirming the death of the Italian and saying that at least one suspect had been arrested.

Dozens of journalists, who knew the activist well, gathered in front of the morgue. The Hamas police allowed only colleagues of Mr. Arrigoni, who were mostly foreigners, inside.

According to a friend who was in touch with him by e-mail, Mr. Arrigoni was abducted on the day he planned to leave Gaza. “I am very tense, exhausted, if they don’t kill anyone in the next 24 hours, I am getting out Thursday. Your V.,” he wrote on Monday to a friend in Italy, Daniela Loffreda, Ms. Loffreda said in a telephone interview.

The abduction was the first kidnapping of a foreigner in Gaza since Hamas, an Islamic militant group, took control of the territory in June 2007. It was likely to embarrass Hamas, which has prided itself on restoring security and ending years of armed chaos in Gaza.

Mr. Arrigoni last entered Gaza in 2009, friends said. Mr. Arrigoni had been an active participant in demonstrations and rallies against a blockade imposed by Israel with Egypt’s help. The restrictions on the entry of goods overland have eased in recent months, but a strict naval blockade remains in force.

Mr. Arrigoni was a native of Bulciago, a small town near Lake Como north of Milan. He wrote occasionally for the Italian left-wing daily Il Manifesto, which also had a blog, Guerrilla Radio, to which he filed dispatches from Gaza. Luigi Ripamonti, the deputy mayor of Bulciago, said that Mr. Arrigoni had been an activist from an early age and that he had worked in Eastern Europe and Africa before embracing the Palestinian cause.

“He was a kid who always helped others,” Mr. Ripamonti told Italy’s Sky 24 Television. He added, “Today we lose an Italian citizen, a citizen of Bulciago, and also a Palestinian citizen, because he had married Palestine.” Mr. Arrigoni’s mother, Egidia Beretta, is the mayor of Bulciago.

“He has never mixed with powerful people,” said Ms. Beretta said in an interview to Italy’s Sky Television. “He lived in an apartment block on the harbor, he used to tell me that at times he could see the Israeli boats from the distance.”

Mr. Arrigoni started traveling the world as soon as he finished college and arrived for the first time in Israel and Palestine in 2002, almost by chance, Ms. Beretta told the Italian news agency ANSA.

In a video from his blog, broadcast on Italian television, Mr. Arrigoni said: “I don’t believe in borders, in barriers, in flags. I think that we all belong, independent of latitude and longitude, to the same family, the human family.”

In a video on YouTube, Mr. Arrigoni said he came from a family of partisans. “My maternal grandparents fought and died against the occupation, another occupation, the Nazi occupation of Italy,” he said.

“Probably for this reason it’s in my blood, my DNA, to push and struggle for freedom and human rights,” he added.

In his last blog post, dated Wednesday , Mr. Arrigoni wrote that four workers had been killed on Tuesday in a tunnel underneath the Rafah border crossing with Egypt. “These tunnels are used to transfer necessary goods that have allowed the survival of the population of Gaza, strangled for four years by the criminal Israeli occupation,” he wrote. The post showed a photo of a man bringing a goat through a tunnel.

The last foreigner kidnapped in Gaza was Alan Johnston, a BBC Gaza correspondent who was captured in March 2007 and held for 114 days. He was released without violence after negotiations between Hamas and his kidnappers, who belonged to a shadowy radical group calling itself the Army of Islam.

Hamas, which won parliamentary elections in 2006, is itself designated as a terrorist organization by Israel, the United States and the European Union. It has cracked down on smaller, more radical Islamic groups in Gaza since it seized control of the area after a brief, factional war against Fatah, its secularist rival.

In a statement released hours before Hamas announced the death of the captured man, the Italian Foreign Ministry said that it had been carrying out “the appropriate steps for every intervention to protect our citizen.”

Hamas officials said in a statement that the house that was stormed Friday morning belonged to a member of the group that released the video. The officials said one suspect had been arrested. The Associated Press reported that a policeman said four people had been arrested in another location in connection with the abduction.