Billionaire tycoon and would-be US president Donald Trump was heckled and called a racist during his opening monologue as he guest-hosted Saturday Night Live.

But the off-stage boo-boy turned out to be comedian Larry David, who moments earlier had performed his own impersonation of Democrat Bernie Sanders, who is also bidding to be his party's choice to run for the White House.

"Larry, what are you doing?" Mr Trump asked on the NBC TV show.

"I heard if I did that I would get 5,000 dollars," said David with a shrug, echoing an offer reportedly made by pro-Hispanic groups protesting at Republican Mr Trump's appearance.

"As a businessman, I can fully respect that," Mr Trump said.

Mr Trump hosted SNL as scheduled, amid high anticipation, sharp criticism and calls for the show to dump him.

Despite a 40-year history of lampooning politicians while inviting some to mock themselves as on-air guests, booking a presidential candidate to host the NBC sketch-comedy show was almost unprecedented.

Previously, only eight politicians had served as guest hosts. Only one of those - the Rev Al Sharpton, in 2003 - was actively involved in a presidential bid at the time. Hillary Clinton appeared in last month's season premiere, but not as the host.

The Saturday TV platform granted Mr Trump fanned the flames of outrage sparked in June when he announced his Republican candidacy for president and described some Mexicans who are in the United States illegally as criminals and rapists.

NBC faced mounting pressure from a coalition of advocacy groups calling for him to be dropped from SNL for what one spokesman termed his "racist demagoguery".

Hours before the show, dozens of protesters marched from Trump Tower to NBC's studio in New York's Rockefeller Plaza, chanting in both English and Spanish and carrying signs. In Spanish, they chanted "The people united shall never be defeated" and signs declared SNL racist.

"I feel like they're giving him a platform," said Hazel Hernandez, 26, who emigrated from El Salvador and now lives in Brooklyn.

"I'm an immigrant myself, so I'm pretty outraged. I've been in this country for many years, and I'm outraged that they would let him host SNL. It's upsetting."

Mr Trump's comments last summer spurred NBC to sever its Miss Universe ties with him while declaring he would never return to his The Apprentice role.

But leading up to Saturday's broadcast, NBC did not respond to the outcry.

It was not Mr Trump's first turn as guest host. The developer and media personality presided in April 2004, a few weeks after he debuted as host of The Apprentice.

Typically outspoken, Mr Trump welcomed the controversy, predicting it would only boost his audience.

For candidate Mr Trump, whose many interview and debate appearances have drawn large audiences, a surge in this week's SNL ratings seemed assured.