ST PAUL, Minnesota (AFP) - John McCain vowed to fight for America as long as he draws breath, in a patriotic pledge to bring political change as he accepted the Republican presidential nomination.

The 72-year-old war hero and free spirit senator offered himself as a warrior to fight one final battle for his country, arguing a character forged in war gave him the judgement and vision to lead from the White House.

"I don't mind a good fight," said McCain , who appeared on stage late Thursday in a white spotlight following a video extolling his heroic story as a Vietnam war prisoner, "a man who found his strength in a tiny dank cell miles from home."

McCain's speech ushered in the critical phase of the US election, a frenetic cross-country marathon including three face-to-face debates, which will decide whether he, or Democrat Barack Obama moves into the White House.

"In the end, it matters less that you can fight. What you fight for is the real test," said McCain, exactly two months from election day.

McCain, who has a reputation of bucking party's orthodoxy, promised he and vice presidential running mate Sarah Palin would shatter the political gridlock in Washington.

"Let me offer an advance warning to the old, big-spending, do-nothing, me-first, country-second Washington crowd: change is coming," McCain said, adding he was honored to accept the party's nomination.

Democrats dismissed McCain as no agent of change and tied him to fellow Republican President George W. Bush, who was rarely mentioned during the party's convention.

"After days of negative attacks -- and no mention of real proposals to fix our economy, get more people health care, or make America safer -- the party that brought you eight years of disastrous policies is asking for four more," Obama's vice presidential pick Joe Biden said in a statement.

In stark contrast to Obama's spectacular convention finale in an open-air football stadium in Denver last week, McCain stood on a simple stage stretching out into seated Republican delegates.

McCain is attempting to co-opt Obama's mantle of change in a year in which polls show Americans overwhelmingly think their country is heading in the wrong direction.

"I'm not running for president because I think I'm blessed with such personal greatness that history has anointed me to save our country in its hour of need," McCain said in a clear swipe against Obama.

"My country saved me. My country saved me, and I cannot forget it. And I will fight for her for as long as I draw breath, so help me God."

His speech was punctuated by standing ovations but there was none of the euphoria which greeted Palin, the new conservative heroine, on Wednesday.

As the traditional red, white and blue balloons tumbled from the rafters, McCain and Palin stood together in a cloud of confetti as the new face of the Republican Party.

McCain mentioned Bush only once, and not by name, as he thanked the unpopular president for preventing another September 11-style attack.

The primetime convention speech watched on television by millions of Americans was briefly interrupted by several anti-war protestors who had sneaked into the crowd.

One held up a black sign reading "You can't win an occupation," and started chanting, but was quickly drowned out by the crowd cheering "USA, USA," and then hustled away by security staff.

Outside the hall, hundreds of anti-war protesters tried to march to the convention but their path was blocked by rows of police in riot gear, who ordered them to disperse.

When protesters refused to do so as ordered, a tense confrontation ensued on Thursday evening with demonstrators sitting down in the street about a mile from the convention hall.

More than 200 people were taken into custody on the Marion Street bridge, the Minneapolis Star-Tribune reported.

Police used tear gas and pepper spray in the clash with demonstrators, and more than a dozen journalists were among those detained, the Star-Tribune said.

In his speech, McCain said he had worked for decades across the political aisle, contrasting his record with that of Obama, a first term senator.

"That's how I will govern as president. I will reach out my hand to anyone to help me get this country moving again.

"I have that record and the scars to prove it. Senator Obama does not."

McCain also attempted to make a connection with everyday Americans who are struggling to make ends meet, with the economy the number one issue in the election.

"I know some of you have been left behind in the changing economy and it often seems your government hasn't even noticed," McCain said.

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