Thousands of Hamas supporters on Saturday called for "revenge" during the Gaza funeral of an official from the Palestinian Islamist movement gunned down the previous day.
"Revenge, revenge!" called participants at the procession for Mazen Faqha, 38, who was shot dead by unknown gunmen in the Gaza Strip on Friday.
Hamas-nominated attorney general Ismail Jaber on Saturday blamed Israel for the killing.
"This assassination has the clear marks of Mossad," he said, referring to the Jewish state's spy agency.
According to Israeli media, Faqha was responsible for cells of Hamas's military wing, the Ezzedine al-Qassam Brigades, in the Israeli-occupied West Bank.
Israel arrested Faqha and sentenced him to prison over suicide attacks that killed hundreds of Israelis during the second intifada, or uprising, between 2000 and 2005.
He was released in 2011 along with more than 1,000 other Palestinians in exchange for Gilad Shalit, an Israeli soldier Hamas had detained for five years, and transferred to Gaza.
Prime minister Ismail Haniyeh and Yahya Sinwar, the new leader of Hamas in the Gaza Strip, headed the procession from the Shifa morgue to the Omari mosque, an AFP photographer said.
Khalil al-Haya, a deputy to Sinwar, promised retaliation.
"If the enemy thinks that this assassination will change the power balance, then it should know the minds of Qassam will be able to retaliate in kind," he said.
Hamas has ruled the Gaza Strip for 10 years.