Nearly 200 Palestinians have been killed and hundreds injured after the Israeli air force launched dozens of air raids on the Hamas-controlled Gaza Strip.

Palestinian health officials said at least 195 people were killed and more than 250 wounded in one of the bloodiest days for decades in the long-running Israeli-Palestinian conflict. Many were members of the security forces of the Islamic group Hamas, but civilians were also killed.

In an indication of how the operation may yet expand, the Israeli defence minister, Ehud Barak, said Israel's air offensive against militant sites in Gaza "will widen as necessary". He told a news conference: "There is a time for calm and there is a time for fighting, and now is the time for fighting. The operation will expand as necessary." It was unclear if this would include a ground offensive.

The Israeli military said it attacked "terrorist infrastructure". Hamas vowed to avenge what it called "the Israeli slaughter" and Palestinian militants responded with rocket salvos that killed an Israeli man and wounded others.

The strikes come just over a week after a six-month ceasefire between Israel and Hamas expired, and follow a series of warnings by Israeli officials that they were planning an operation in response to rockets fired into Israeli towns and villages by Hamas and other militant groups based in Gaza. More than 50 rockets have been launched from Gaza in recent days, according to Israeli military officials. Israel yesterday reopened crossings into the Gaza Strip, allowing in humanitarian aid after an eight-day closure, in what has been seen as an attempt to pre-empt international criticism ahead of today's attacks.

Despite the warnings, the timing and scale of the assault came as a surprise to many residents of Gaza. The raids had been expected to begin tomorrow, and the fact that they took place mid-morning rather than at night meant many official buildings and schools were full. Some of the missiles struck densely populated areas as children were leaving school. Parents rushed into the streets to search for them.

Television footage from Gaza showed bodies scattered on a road and the dead and wounded being carried away. Civilians rushed to the targeted areas and tried to move the wounded in their cars to hospital.

Hamas said it would seek revenge, including launching new rocket attacks on Israel and sending suicide bombers to Israel. "Hamas will continue the resistance until the last drop of blood," said a Hamas spokesman, Fawzi Barhoum, speaking on a Gaza radio station.

Residents reported hearing two waves of explosions, with at least 15 blasts in the first wave. Plumes of black smoke billowed over Gaza, where the dead and wounded lay scattered on the ground after air strikes destroyed Palestinian security compounds, including two where Hamas was hosting graduation ceremonies for new recruits.

One man sat in the middle of a street in Gaza City close to a security compound, slapping his face and covering his head with dust from the bombed-out building. "My son is gone, my son is gone," said Sadi Masri, 57. The shopkeeper said he sent his son out to buy cigarettes minutes before the air strikes and could not find him. "May I burn like the cigarettes, may Israel burn," Masri said.

International reaction has been swift, with many world leaders calling for an immediate ceasefire. "We are very concerned at the events in Gaza. We call for an immediate ceasefire and urge everybody to exert maximum restraint," said a spokesman for Javier Solana, the EU foreign policy chief.

The Foreign Office said it was "deeply concerned by the reports of deaths and injuries of innocent civilians" in Gaza. A spokesman said: "The only way to achieve peace in Gaza is through peaceful means. While we understand the Israeli government's obligation to protect its population we urge maximum restraint to avoid further civilian casualties. We also call on militants in the Gaza Strip to immediately cease all rocket attacks on Israel."

The United States urged Israel to avoid civilian casualties, although it did not call for an end to the Israeli strikes. "Hamas's continued rocket attacks into Israel must cease if the violence is to stop. Hamas must end its terrorist activities if it wishes to play a role in the future of the Palestinian people," said a White House spokesman, Gordon Johndroe. "The United States urges Israel to avoid civilian casualties as it targets Hamas in Gaza."

Arab foreign ministers will meet in Cairo tomorrow or Monday to take a common position on the Israeli raids. Libya, the only Arab country on the UN security council, would seek an urgent meeting, said the Arab League secretary general, Amr Moussa. "We are facing a continuing spectacle which has been carefully planned. So we have to expect that there will be many casualties. We face a major humanitarian catastrophe," Moussa said.

In the West Bank, the moderate Palestinian president, Mahmoud Abbas, said in a statement that he "condemns this aggression" and called for restraint.

In a statement, the Israeli Defence Force (IDF) said the attacks were in retaliation for "the continuation of terrorist activity by the Hamas terror organisation from the Gaza Strip, and the continuation of rocket launching and targeting Israeli civilians". The strikes were based on recent intelligence and aimed at Hamas terror operations emanating from its headquarters, training camps and weapons warehouses, according to the IDF statement.