The French presidency of the European Union and the office of the EU's foreign policy chief called for an immediate ceasefire in Gaza.
"We are very concerned at the events in Gaza. We call for an immediate ceasefire and urge everybody to exert maximum restraint," a spokesman for European Union foreign policy chief Javier Solana said.
"Everything must be done to renew the truce," he added.
Israeli war planes and combat helicopters pounded the Hamas-ruled Gaza Strip on Saturday, killing at least 155 people and prompting rocket fire from Palestinian militants that killed an Israeli, medics said.
'Only the beginning'
Columns of black smoke could be seen rising from locations across the territory on Saturday, and artillery fire was reported against Palestinian militants firing rockets into Israel.
The bombardment is "only just the beginning," Israeli military spokesman Avi Benayahu told army radio on Saturday.
"This is only just the beginning of an operation launched after a security cabinet decision. It could take time. We have not fixed a timeline and we will act according to the situation on the ground," he said.
Hamas has vowed to avenge what it called "the Israeli slaughter."
Arabic television station al-Jazeera broadcast pictures showing the motionless bodies of a number of young men lying on the ground in Gaza. Footage showed the wounded and dead being carried away by rescuers. There was widespread damage to buildings.
Medics and witnesses are reporting at least 155 people have been killed in the bloodiest day for Palestinians in more than 20 years.
Among the dead was the Gaza police chief, Maj. Gen. Tawfiq Jaber, witnesses told reporters. He was in a police compound in Gaza City that had been hosting a graduation ceremony for new personnel when it was attacked.
Airstrikes also paralyzed the mobile telecommunications network in the territory.
Earlier than expected
The attacks, although an expected response to a wave of rocket fire by the military associates of the Islamist Hamas movement that has ruled Gaza since the end of a ceasefire on Dec. 19, came as a surprise on Saturday, as previous Israeli media reports had not expected a decision to strike before Sunday.
Palestinian security sources told the DPA news agency that the airstrikes targeted police and security installations run by the Hamas movement.
Witnesses said that Israeli artillery fire struck at Palestinian militants who had fired rockets into Israel west of the town of Beit Lahiya, in the north of the Gaza Strip.
"We are facing a period that will not be simple or easy," an aide to Defense Minister Ehud Barak told Reuters.
Three members of Hamas' military wing were injured by shrapnel, Palestinian emergency official Mu'awia Hassanien said.
Arab foreign ministers will meet in Cairo on Sunday or Monday to take a common position on Israeli raids, the head of the Arab League said on Saturday.