Gunmen from the Palestinian group Hamas rampaged through Israeli towns on Saturday, killing at least 250 Israelis and escaping with dozens of hostages in by far the deadliest day of violence in Israel since the Yom Kippur war 50 years ago.

More than 230 Gazans were also killed when Israel responded with one of its most devastating days of retaliatory strikes. "We will take mighty vengeance for this wicked day," Israel Prime Minister Benjamin Netanyahu said.

The horrific and escalating violence is one of the many extreme cases of fighting between Hamas and Israel.

Hamas is a Palestinian militant group which rules the Gaza Strip and wants to replace Israel with an Islamic state. Hamas has fought several wars with Israel since it took power in Gaza in 2007.

Israel, the United States, the European Union and the UK, as well as other powers, have labelled Hamas a terrorist group.

What is Hamas?

Hamas is a Palestinian Islamist militant group which rules the Gaza Strip. Hamas militant commander Mohammed Deif recently called on Palestinians and other Arabs to join the militants' operation to "sweep away the [Israeli] occupation".

It was created in 1987 during an uprising against Israel's occupation of Gaza and the West Bank. Its name is an acronym of an Arabic phrase that translates as Islamic Resistance Movement.

Hamas or its military wing, the Al Qassam brigades, are designated terrorists by Israel and most Western nations, including the UK, the US, Canada and the entirety of the European Union.

It is also, however, seen as the legitimate administration in the territory by countries including Iran, which is one of the group's major backers, and the Shia Islamist group Hezbollah, in Lebanon.

Iranian security officials helped plan Hamas’s surprise attack on Saturday and gave the green light for the assault at a meeting in Beirut last Monday, according to senior members of Hamas and Hezbollah, another Iran-backed militant group.

'The most serious attack Israel has faced in more than a generation'

The BBC's International Editor Jeremy Bowen has said: "This is the most ambitious operation Hamas has ever launched from Gaza and the most serious cross-border attack Israel has faced in more than a generation.

"Militants breached the wire that separates Gaza from Israel in multiple places.

"The unprecedented attack came a day after the 50th anniversary of the surprise attack by Egypt and Syria in 1973 that started a major Middle East war. The significance of the date will not have been lost on the Hamas leadership."

The Gaza Strip is a 41km (25-mile) long and 10 km-wide territory between Israel, Egypt and the Mediterranean Sea. It is home to about 2.3 million people and has one of the highest population densities in the world.