Forty were injured in the bus explosion in Haifa
Fifteen people were killed and 40 wounded by a suicide bombing on a bus in Haifa at lunchtime.
In its first response, the Israeli Government imposed a blockade on Palestinian territories; Prime Minister Ariel Sharon is returning early from a visit to the United States.
The Palestinian Authority also convened a security meeting and declared martial law in the West Bank and Gaza Strip.
The new measures allow the prevention of demonstrations and detention without due procedure, and any movement not committed to the authority's decisions will be outlawed.
The violence has wrecked an American peace mission to the region, BBC correspondents say.
US Secretary of State Colin Powell described this as a "moment of truth" for Palestinian leader Yasser Arafat to rein in the attacks.
Teenagers killed
The violence began on Saturday night at a shopping centre in the Ben Yehuda precinct of Jerusalem.
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WEEKEND ATTACKS:
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Ten people were killed and 170 injured, most of them teenage revellers.
Israeli Foreign Minister Shimon Peres described the attack as "one of the worst... ever seen".
He summoned all foreign ambassadors in the country to a meeting to impart the "extreme gravity of the situation."
Israeli officials blamed Palestinian leader Yasser Arafat, and the United States demanded that he act against those responsible for the attacks.
As the violence continued into Sunday, Palestinian gunmen infiltrated a Jewish settlement in northern Gaza, killing at least one Israeli and wounding five others.
The gunmen were later killed by Israeli forces.
Israeli Prime Minister Ariel Sharon has warned of a "commensurate" response.
His talks with President George W Bush have been brought forward by 24 hours to Sunday.
Mr Bush demanded that Mr Arafat "act swiftly and decisively" against the organisations that support those who carried out the attacks.
US Defence Secretary Donald Rumsfeld, meanwhile, said the balance had changed in the region as a result of the attacks.
"The only way to defend against terrorists is to go after the terrorists," he said, appearing to back Israel's controversial policy of targeted assassinations of individuals who threaten its citizens.
Country shaken
Saturday night's blasts went off in quick succession in a busy Jerusalem neighbourhood packed with restaurants and cafes, shortly after midnight local time.
Twenty minutes later, a car bomb explosion in a nearby side street sent screaming survivors running in all directions.
The area was particularly busy, following the end of the Jewish Sabbath.
A witness, Eli Shetreet, told the Associated Press news agency he had seen bodies hurled in the air by the blasts.
"People were crying, falling, and there was the smell of burning hair," said Shetreet, 19.
Blood was splattered across shop windows, with pieces of flesh and metal strewn on the ground.
Blame and denial
An Israeli Government official said Mr Arafat was guilty of a "total lack of action in the domain of fighting terror".
But the Palestinian leadership issued a strongly-worded statement, condemning the attacks and pledging to track down those responsible.
The militant group Hamas, on its website, claimed responsibility for the Jerusalem and Haifa attacks.
Hours after the Jerusalem blasts, Israeli forces arrested several alleged militants in the village of Abu Dis on the outskirts of the city.
Palestinian police arrested a top member of Islamic Jihad, Mohammed al-Hindi after a gun battle at his Gaza home on Saturday.
Hamas and fellow militants Islamic Jihad have threatened to avenge Israel's killings of prominent members of militant groups.
Condemnation and messages of sympathy came from around the world, with Jordan also urging Israel to show restraint in the interests of peace.
Egypt strongly condemned the attacks, as did France, Russia and many other countries.
US demand
Senior US envoy Anthony Zinni - visiting Israel and Palestinian-controlled areas to try to secure a ceasefire - is reported to have been heckled by angry Israelis as he laid a wreath to the victims of the bombings.
"Mind your own business," they shouted at him. "Go home."
Mr Zinni urged Mr Arafat to hunt down the organisers of the attacks.