Tens of thousands of U.S. conservatives converged on Washington Saturday for a controversial rally on the steps of the Lincoln Memorial, the site of Rev. Martin Luther King Jr.'s famous "I Have a Dream" speech.

Civil rights leaders denounced the decision to hold the rally on the same day and the very spot where King delivered his speech 47 years ago and held a rival event to honour King.

But right-wing broadcaster Glenn Beck, the radio and television talk show host who organized the "Restoring Honour" rally, insisted that the timing and location of the event was a coincidence.

Beck told the vast, mostly white crowd on Saturday that he was humbled by the turnout. The crowd stretched for hundreds of metres along the Washington Mall's long reflecting pool.

"For too long, this country has wandered in darkness," said Beck, a Fox News host, saying it was time to "concentrate on the good things in America, the things we have accomplished and the things we can do tomorrow."

"Something beyond imagination is happening," he said. "America today begins to turn back to God."

The rally's marquee speaker, former Republican vice-presidential candidate Sarah Palin, was greeted by chants of "USA, USA, USA" from many in the crowd.

"It is so humbling to get to be here with you today, patriots. You who are motivated and engaged ... and knowing never to retreat," said the former Alaska governor. "We must restore America and restore her honour."

She added: "We must restore America and restore her honor."

Beck is known for his controversial views and statements, once describing President Barack Obama as a racist. But both he and Palin steered clear of politics in their speeches.

Palin told the crowd she wasn't speaking as a politician. "I've been asked to speak as the mother of a soldier and I am proud of that distinction. Say what you want to say about me, but I raised a combat vet and you can't take that away from me."

Palin's son Track, 20, served a year in Iraq.

While the rally's organizers billed the event as non-political, the show of force from conservatives comes before the run-up to mid-term elections.

Palin honoured the U.S. military in her speech and likened the rally participants to the civil rights activists who came to the National Mall to hear King's historic speech. She said the same spirit that helped civil rights activists overcome oppression, discrimination and violence would help this group as well.

But some civil rights leaders objected to the timing and venue of the rally and organized their own march Saturday to the site of a proposed King memorial not far from the Lincoln Memorial.

Rev. Al Sharpton called the rally an anti-government demonstration, something that goes against the message in King's speech, in which the civil rights leader appealed to the federal government to ensure equality.

Sharpton's counter-rally took place at a nearby high school, and those in attendance marched through the capitol and briefly came into contact with participants at the conservative event, some who wore tea party T-shirts.

As Sharpton's group shouted "reclaim the dream" and "MLK, MLK," some members of the conservative rally shot back.

One woman shouted: "Go to church. Restore America with peace."

However, both crowds were generally well behaved.

"It is ironic to me that they come on the day of a speech where Dr. King appealed for a strong government to protect civil rights and they're going to the site of Abraham Lincoln who saved the union against the state rebellion," Sharpton said Saturday.

While an official crowd count was not immediately known, Beck's conservative organizers had obtained a permit for 300,000.

King's historic speech came at a critical point in black Americans' struggle for civil rights and equality under the law.

"I have a dream that my four little children will one day live in a nation where they will not be judged by the colour of their skin but by the content of their character," King told a massive audience that jammed the National Mall below the memorial to Abraham Lincoln on Aug. 28, 1963.

King was assassinated in Memphis, Tennessee, five years later.

With files from The Associated Press