Israeli intelligence officials have said a bus bomber who injured at least 21 people in Tel Aviv was a lone operator with no links to any major groups, according to Sky sources.
The explosion took place across from the military headquarters - on the eighth day of an Israeli offensive against Palestinian militants in Gaza - and hours before a ceasefire was announced .
Ofir Gendelman, a spokesman for Israeli Prime Minister Benjamin Netanyahu, immediately condemned the explosion as a "terrorist attack".
Hamas spokesman Sami Abu Zuhri praised the bombing, but stopped short of claiming responsibility.
"Hamas blesses the attack in Tel Aviv and sees it as a natural response to the Israeli massacres ... in Gaza," he said.
Police said it was not a suicide attack and suggested an explosive may have been planted on the No 142 bus as forensic teams took away bomb fragments for analysis.
Israeli Police spokesman Micky Rosenfeld told Sky News: "We have heightened security all around the Tel Aviv area in order to also see if there are any suspects that fled the area."
Unconfirmed reports from Israel said police were holding a man caught running away from the scene moments before the bombing, and were looking for a woman who was on the bus earlier.
The driver, who escaped largely unscathed, told reporters he had not seen anyone suspicious get on board. "I felt the explosion ... smoke was everywhere, you couldn't see a thing," he said.
Passenger Yehuda Samarano, 59, who suffered shrapnel wounds to his chest and leg, said from his hospital bed: "I flew from my seat. Everything became white and my ears are still ringing now."
The blast happened at around noon in one of the coastal city's busiest areas, near the Tel Aviv Museum, business hub, diamond district and an entrance to the Kirya, Israel's national defence headquarters.
Television footage showed pictures of a smoke-filled bus, charred inside with its windows blown out.
Leor Sinai, a resident who visited the scene after the explosion, said there was "chaos, mayhem".
He told Sky News: "Thankfully, there's a hospital around the corner so the people were brought right to the hospital. They were, from what I hear, hit with nails, that the bomb was filled with nails and little sort of marbles that kind of flew in all different directions."
Medics said three of the wounded were in a moderate-to-serious condition. Some reports suggested up to 17 or 21 people had been injured in the blast.
Israel has been locked in a deadly week-long confrontation with Palestinian militants in Gaza after an Egypt-brokered truce fell through.
But an Egyptian-brokered ceasefire, to begin on Wednesday evening, has now been confirmed.
British Foreign Secretary William Hague said the bus explosion was of "deep concern".
The White House also condemned the attack as an "outrageous" assault on "innocent Israeli civilians".
Hamas militants have fired at least four rockets at Tel Aviv in the past week, but none have resulted in direct hits or any casualties.
The last time the city was hit by a significant bomb blast was in April 2006, when a Palestinian suicide bomber killed 11 people at a sandwich stand near the city's old central bus station.