Charles traveled to New York with his wife Camilla, the Duchess of Cornwall, to receive the Global Environmental Citizen award from the Harvard Medical School's Center for Health and the Global Environment.

American actress Meryl Streep and last year's winner, former US vice president Al Gore, presented Charles the award for leadership in environmentally friendly urban design.

But green activists in Britain criticised the royal couple's decision to fly across the Atlantic with a 20-person entourage, accusing the prince of 'green hypocrisy'.

"Why can't he set a genuine example and accept the award by video link instead?" Sian Berry, the principal speaker for Britain's Green Party, was quoted as saying in Britain's Independent newspaper.

"I am disappointed to see that ... his recognition of the threat posed by climate change does not extend to toning down his personal flying habits," Berry was quoted as saying.

The royals arrived in the United States on a British Airways commercial flight on Friday, kicking off a US tour devoted to urban renewal and conservation in Philadelphia.

The unusual mode of travel, the prince usually flies by private jet, was presented as a more environmentally sensitive approach.

The royal couple, last in New York in November 2005 when they visited the site of the September 11 attacks, worshipped early Sunday at a Philadelphia Presbyterian Church before taking a private train to New York.

Charles, wearing a dark blue double-breasted suit, and Camilla, in a brown, cloth coat with a beaded collar and leopard-print cuffs, paid a visit to Harlem Children's Zone, which runs a school and a range of social and educational services.

At a class that teaches kids about finances with virtual stock investments, one pupil, Tiffany Vargas, 13, asked the prince if his organic food company was listed on the stock exchange.

"No, it's still a private company. We haven't got quite to that stage yet," he said.

The couple later attended a rehearsal with youths performing a scene from Shakespeare's 'A Midsummer Night's Dream'.

Charles, asking a 12-year-old actor how he remembered his lines, joked: "It's much easier when you're young. I can't remember anything now, it's terrible."

The couple then watched a basketball practice. After witnessing a shooting drill, Charles was presented with a ball and managed to sink a basket on his second try.

On Saturday, Charles, 58, and his wife, Camilla, 59, motored to Philadelphia's Independence Hall, where Americans declared their independence from British rule in 1776, escorted by the First Troop Philadelphia City Cavalry in full military garb.

The future king and Camilla received an enthusiastic welcome in the first visit to Philadelphia by a prince of Wales in more than a century and a half.

"Never in a million years did I think we'd have the opportunity to meet the Prince of Wales," said Lisa Diane, 40, of New Jersey, who got a welcome surprise when Charles shook her hand.

"Out of all the places for him to visit ... this is phenomenal, and it makes history for us."

After visiting the 250-year-old Liberty Bell and Independence Hall, they traveled to a West Philadelphia neighbourhood for a firsthand look at the city's revitalisation efforts in the form of one of the many elaborate murals scattered throughout the city.

In the evening, the couple attended a ball at the city's elegant Academy of Music, with British rocker Rod Stewart headlining the event.