The new leader of the Palestinian militant group Hamas in Gaza has denounced President George Bush as an "enemy of Muslims".
Mr Rantissi is regarded as a hardliner and a firebrand orator
Abdel Aziz al-Rantissi was speaking to some 5,000 supporters in Gaza City.
He said Mr Bush and Israeli Prime Minister Ariel Sharon had "declared war against Allah" but "Allah declares war against America, Bush and Sharon".
Mr Rantissi was named leader in Gaza after Israel assassinated Hamas founder Sheikh Ahmed Yassin last Monday.
Sheikh Yassin, the group's spiritual head, was killed in an Israeli air strike as he returned from a mosque in Gaza City at daybreak.
Seven others were killed and many wounded in the attack.
Israel accused Sheikh Yassin of ordering Hamas suicide bombings which killed hundreds of Israelis.
In the wake of his death, the words of Hamas leaders are being watched very closely for any sign of a further hardening of the organisation's line, or that it might even strike at targets in the West, says the BBC's Alan Johnston in Gaza.
But long-time observers of Hamas in Gaza say that it would be wrong to read too much into Dr Rantissi's speech as he and other Hamas leaders have made this kind of rhetorical attack on the US many times before, our correspondent reports.
Hamas has always made it clear that its military target is Israel and Israel alone.
Speaking at the Islamic University in Gaza on Sunday, Mr Rantissi said he was not surprised that the US vetoed a UN Security Council resolution condemning the missile strike which killed Sheikh Yassin.
"[Sheikh Yassin] said the war of God continues against them and I can see the victory coming up from the land of Palestine by the hand of Hamas", he said.
Israeli government spokesman Avi Pazner has described Mr Rantissi as "one of the most extreme members of Hamas".
Questioned over the killing of Sheikh Yassin last week, President Bush said that Israel had a "right to defend itself from terror" - but should also bear in mind the consequences.
Following the sheikh's assassination, President Bush reiterated his support for a two-state solution to the Israeli-Palestinian dispute and for a concerted fight against terror.
Mr Rantissi, a 56-year-old doctor, has been imprisoned in the past by both Israel and by Yasser Arafat's Palestinian Authority.
In 2003, he survived an Israeli assassination attempt similar to that which killed Sheikh Yassin.