A man spent three hours scaling Trump Tower, the New York home and headquarters of the Republican nominee for US president, before police captured him through an open window Wednesday.

Live television footage showed uniformed New York police officers reaching out and grabbing the young man -- dressed in grey shorts, an olive T-shirt and white cap -- around three hours after he started his ascent using five suction cups.

The man with curly auburn hair had been edging his way up the side of the building gingerly -- at times with his feet dangling precariously as he gripped the suction cups -- on a hot and steamy afternoon.

New York billionaire Donald Trump was outside the city on Wednesday, holding a rally in Virginia with another event planned later in Florida.

"The climber has been taken into custody," tweeted Sergeant Carlos Nieves from the police public information office.

The climber, whose identity and motive was not immediately known, was watched by a crowd of mesmerized onlookers, as well as reporters and police, who cordoned off the block between Fifth and Madison Avenues, an AFP reporter said.

In an anonymous video posted on YouTube, entitled "Message to Mr Trump (why I climbed your tower)," a young man dressed in a black hoodie with his face partially obscured exhorted Americans to vote for the Republican in November.

"The reason I climbed your tower was to get your attention," said the man, addressing the White House candidate and identifying himself as "an independent researcher."

"As for anyone else who's watching, please help make this video go viral so that it gets to Mr Trump and be sure to get out and vote for Mr Trump in the 2016 election," the young man added.

- Earlier evasion -
There was no immediate confirmation about the authenticity of the video.

Authorities removed two windows from the 21st floor and it was there that officers grabbed him about 6:35 pm (2235 GMT), after starting his ascent from a terrace area on the fifth floor.

The climber had earlier evaded police by climbing around an opened window and washer scaffold. Police deployed airbags in case he fell.

Trump lives in a marble triplex at the top of the 68-story building, which is also the headquarters of his Trump Organization and election campaign.

The Republican nominee unleashed a firestorm on Tuesday when he suggested that "Second Amendment people" -- those who support gun rights -- could take action to stop Hillary Clinton from appointing US Supreme Court justices as president.

Since he launched his controversial, media-saturated bid for the White House, the tower on New York's prestigious Fifth Avenue has become a magnet for tourists.

Members of the public are free to roam inside its public atrium until 10 pm most days as part of an agreement that allowed Trump to build 20 stories higher.