Israel has released Mohammed Taha, a co-founder of the Palestinian militant group Hamas, after 14 months in detention without trial.
Thousands poured into the streets to welcome Taha home
Mr Taha, now 68, was driven to a hero's reception in the Bureij refugee camp in central Gaza, where he was paraded on supporters' shoulders.
Israel, which has killed two senior Hamas figures in recent weeks, did not comment on the release.
Later, Israeli soldiers killed another Hamas leader in the northern West Bank.
Imad Janajra, 30, was killed in the village of Taluza, north of Nablus, the security sources and witnesses said.
An Israel military source told Reuters news agency that Mr Janajra was shot when soldiers spotted him armed and approaching them.
He was reported to be the leader of Hamas' armed wing, the Ezzedin al-Qassam Brigades.
Mr Taha was reportedly released at the Erez crossing in northern Gaza on Wednesday afternoon and driven to his home in the Bureij refugee camp in central Gaza.
Thousands of people streamed out of their homes to celebrate Mr Taha's unexpected freedom as mosques broadcast calls for celebrations in the street.
Hamas activists crowded around Mr Taha's car, some firing their assault rifles into the air, before he was lifted onto supporters' shoulders.
Mr Taha said targeted killings would not crush the Palestinian spirit
Mr Taha was arrested in March 2003, during an Israeli incursion into the Bureij camp.
His five sons, also Hamas activists, were also arrested.
They alleged at the time Mr Taha was beaten by Israeli soldiers until he fainted.
They were all subsequently freed, although one son, Yasser, was killed along with his wife and two-year-old daughter in a targeted air strike on their car in June.
Two senior Hamas leaders have recently been killed in such targeted strikes: the group's spiritual leader Sheikh Ahmed Yassin on 22 March and Hamas' leader in Gaza, Abdel Aziz Rantissi, three weeks later.
Commenting on the death of Rantissi, Mr Taha said: "It is the will of God. If the enemy thinks that by killing our leaders they can kill our spirit and determination, they are mistaken."
Hamas has claimed responsibility for dozens of the suicide attacks which have killed hundreds of Israelis in the three-and-a-half years of the intifada (uprising).