The spiritual leader and founder of Hamas, Sheikh Ahmed Yassin, has been killed in an Israeli air strike in Gaza. At least seven others also died in the attack, including two of his bodyguards.

Sheikh Yassin died instantly after being struck by missiles from Israeli military helicopters as he left a mosque near his home in Gaza City, early Monday.

Following the attack, senior Hamas official Abdul Aziz Rantisi joined thousands of Palestinians who marched through Gaza calling for revenge. He says that the killing of Sheikh Yassin means that Hamas will now start targeting Israeli political figures.

"The road is opened," he said. "It is an open war, I know that they [Hamas members] will succeed in assassinating the political leaders [in Israel]."

Israel is taking the threats seriously, imposing a closure on the Gaza Strip. Military roadblocks have also been established to prevent free movement and effectively dividing the territory into three parts.

The Gaza Strip is the main stronghold of Hamas, which is listed by the U.S. State Department as a terrorist organization.

Sheikh Yassin, 67, a wheelchair-bound cleric, is the most senior leader of a militant organization to be have been assassinated by Israel since the start of a Palestinian uprising, more than three years ago.

Senior Palestinian officials, including Cabinet Minister Ghassan Khatib, say his killing is only likely to escalate the conflict with Israel. "This is another Israeli political assassination that all the Palestinian people and the Palestinian Authority condemn and look at as a crime," he says.

However, Israeli officials say the killing of Sheikh Yassin was justified.

The officials say Sheikh Yassin had been an inspiration to members of the military wing of Hamas, which has carried out frequent suicide bombings and other terror attacks that have killed hundreds of Israelis.

The killing of Sheikh Yassin comes just one day after Israeli Defense Minister Shaul Mofaz warned that plans had been drawn up to target leaders of Hamas.

Mr. Mofaz says that Israel wants to weaken Hamas before a proposed withdrawal of troops and Jewish settlements from the Gaza Strip.

Israel launched a military offensive against Hamas, last week, following a double-suicide bombing in Ashdod that killed 11 Israelis.