At the conclusion of his first trip to Jerusalem as Premier, Mr Brown vowed that Britain would stand by Israel's side as it faced threats to its existence.
Condemning as "abhorrent" Iranian president Mahmoud Ahmadinejad's call for Israel to be wiped from the map, he said he would not stand by as Tehran sought to acquire nuclear weapons.
The Prime Minister also urged Israel to seize the opportunity of lasting peace with the Palestinians.
He was heckled at one point as he raised again the need for Israelis to withdraw from settlements on Palestinian land - an issue over which he clashed with his opposite number, Ehud Olmert, yesterday.
Mr Brown, who was greeted at the Knesset with a red carpet and guard of honour, pleased his audience of MPs and visitors with a Hebrew phrase soon into his speech, "Shalom aleichem", meaning peace be unto you.
He went on to describe his long-standing admiration for Israel, stemming from his churchman father's fascination with, and frequent visits, to the country.
He said his premiership would ensure that Britain remained Israel's "true friend".
"A friend in difficult times as well as in good times, a friend who will stand beside you whenever your peace, your stability and your existence are under threat, a friend who shares an unbreakable partnership based on shared values of liberty, democracy and justice.
"And to those who mistakenly and outrageously call for the end of Israel, let the message be: Britain will always stand firmly by Israel's side."
He said anti-Semitism and persecution was to be condemned in "all its forms", adding: "To those who believe that threatening statements fall upon indifferent ears we say, in one voice, that it is totally abhorrent for the president of Iran to call for Israel to be wiped from the map of the world.
"And I promise that just as we have led the work on three mandatory sanctions resolutions of the UN, the UK will continue to lead - with the US and our EU partners - in our determination to prevent an Iranian nuclear weapons programme.
"The EU has gone beyond each of these resolutions. Last month we took action against an Iranian bank involved in proliferation.
"We stand ready to lead in taking firmer sanctions and ask the whole international community to join us.
"Iran has a choice to make: suspend its nuclear programme and accept our offer of negotiations or face growing isolation and the collective response not of one nation but of many nations."
Turning to the Middle East peace process, Mr Brown insisted that a lasting settlement was within reach.
But he risked angering some Israelis by reprising his controversial comments of yesterday when he warned that the continued expansion of settlements into Palestinian areas was undermining the negotiations.
Israeli PM Mr Olmert pointedly acknowledged Mr Brown's criticism following talks last night, but gave no indication that policy would change.
At least one onlooker called out during today's speech as Mr Brown revisited the issue. It was not immediately clear whether the heckler was an MP or a visitor.
Mr Brown said Israel must "seize the opportunity" opened by the celebrated Annapolis peace conference in the US last year.
And he went on: "Because I believe this historic, hard-won and lasting peace is within your reach, I urge you to take it by the hand.
"And to deliver this peace, it is vital also that both sides now create the conditions for a final agreement - the Palestinians acting with persistence and perseverance against the terrorists who attack your country, Israel freezing, and withdrawing from, settlements.
"Like many of your friends, I urge you to make these decisions."
Liberal Democrat leader Nick Clegg said: "While the Prime Minister is absolutely right to condemn President Ahmadinejad's vile threats to Israel, he must come clean about his position on military action against Iran.
"He must make it clear that Britain will not give its approval to a unilateral strike by Israel. This would have devastating consequences for the region, and for our troops in Iraq and Afghanistan.
"Last time the US and UK gave a nod and a wink to Israel over its attack on Lebanon, it went disastrously wrong.
"As Foreign Secretary, Jack Straw said a military strike on Iran would be 'completely nuts'. Gordon Brown must be equally explicit with both Israel and the British public that he will not back unilateral military action against Iran in any form.
"With the US at long last becoming engaged in a process of active diplomacy, now is not the time to be backing away from a peaceful solution."
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