SYDNEY -- A final decision whether Australia will join a U.S.-led attack on Iraq will be made by the government and debated by the Parliament afterwards, Prime Minister John Howard said Sunday.

He promised Parliament could and would be recalled at any time to debate the issue, which brought thousands of anti-war protesters into the streets of Australia's major cities over the weekend.

"The decision is one for the executive government," Howard told Channel Nine's Sunday program. "But I have said, as my predecessors have said, if we take the decision to commit forces, we would have a parliamentary debate and I would want to have a parliamentary debate quickly if there were any decision."

The Labor opposition expressed disappointment on hearing Parliament would debate the issue after a decision was made and not before.

"Our view is that there can be no graver decision than to send our sons and daughters abroad to war, in particular, a war against Iraq," opposition Foreign Affairs Spokesman Kevin Rudd said Sunday. "It is just the right thing to do have first a full parliamentary debate in this country prior to the decision to send to war, not afterwards when it's too late."

Some 20,000 anti-war protesters, many of them Middle-Eastern migrants, took to the streets Saturday and Sunday in a campaign to rally opposition to Australian involvement in another war in Iraq.

More than 10,000 marched in Sydney Saturday in a demonstration organized by an alliance of left-wing activists, trade unions and church leaders and attended by showbusiness personalities and public figures, AFP reported.

Children carrying anti-war placards led a rally of 5,000 through Melbourne, stopping traffic in the city center Sunday. Thousands more attended smaller rallies in Adelaide, Canberra and Brisbane.

Clothing union official Michelle O'neil told the Melbourne rally a U.S.-led push for war was about oil and plans to control the Middle East.

"This will be about the deaths of thousands of innocent people who are no different from you and I in what they care about," she said. "It's about people who have a right to a safe life, about people who have a right to a country that is not invaded when they have not taken action against the U.S."

Howard also warned Australia would be prepared to attack a neighboring country if there was evidence a terrorist act against Australia was being planned.

"I think any Australian prime minister would," he said.

"It stands to reason that if you believe somebody was going to launch an attack on your country, either of a conventional kind or a terrorist kind, and you had a capacity to stop it and there was no alternative other than to use that capacity, then of course you would have to use it. "That situation hasn't arisen because nobody has specifically threatened to attack Australia."

Howard said the UN charter on self-defense, developed when conflicts were defined in terms of nations attacking nations, should be altered to allow nations to preemptively strike at known terrorist threats anywhere.