Two Palestinians stormed a Jerusalem synagogue, attacking worshippers praying inside with knives, axes and guns, killing four people before they were shot dead by police.
Today's attack, the deadliest in Jerusalem in years, is bound to ratchet up fears of sustained violence in the city, already on edge amid soaring tensions over a contested holy site.
Prime Minister Benjamin Netanyahu vowed that Israel will "respond harshly" to the incident, describing it as a "cruel murder of Jews who came to pray and were killed by despicable murderers."
US Secretary of State John Kerry denounced it as an "act of pure terror and senseless brutality and violence."
Israeli police called it a terrorist attack and said the two Palestinian assailants were cousins from east Jerusalem.
It was not immediately known if they were affiliated with any group. Hamas, the militant Palestinian group that runs the Gaza Strip, praised the attack but stopped short of claiming responsibility.
Police spokesman Micky Rosenfeld said six people were also wounded in the attack, including two police officers. Four of the wounded were reported to be in a serious condition.
Footage from the scene showed the synagogue, in Jerusalem's ultra-Orthodox Har Nof neighborhood, surrounded by police and rescue workers following the attack.
Wounded worshippers were being assisted by paramedics and a bloodied butcher's knife lay near the scene of the attack.
"I tried to escape. The man with the knife approached me. There was a chair and table between us ... my prayer shawl got caught. I left it there and escaped," Yossi, who was praying at the synagogue at the time of the attack, told Israeli Channel 2 TV. He declined to give his last name.
Yosef Posternak, who was at the synagogue at the time of the attack, told Israel Radio that some 25 worshippers were inside when the attackers entered.
"I saw people lying on the floor, blood everywhere. People were trying to fight with (the attackers) but they didn't have much of a chance," he said.
Police spokeswoman Luba Samri said the attackers were Palestinians from east Jerusalem, which has been the scene of relentless clashes between Israeli police and Palestinian protesters in recent months.
She identified the assailants as Ghassan and Oday Abu Jamil from the Jabal Mukaber neighborhood.
Soon after the attack, dozens of police officers gathered outside the Abu Jamils home. Samri said this was part of the police investigation. She said residents threw stones at the police officers. Israel has been on edge with a spate of attacks by Palestinians against Israelis, killing at least six people in Jerusalem, the West Bank and Tel Aviv in recent weeks, prior to Tuesday's attack.
Kerry blamed the attack on Palestinian calls for "days of rage," and said Palestinian leaders must take serious steps to refrain from such incitement.
"Innocent people who had come to worship died in the sanctuary of a synagogue.
"They were hatcheted, hacked and murdered in that holy place in an act of pure terror and senseless brutality and murder," Kerry said.
Hamas' statement praised the synagogue attack, saying it was a "response to continued Israeli crimes, the killing, desecrating al-Aqsa (mosque)," a reference to a recent incident at the holy site.