Trump booed as chaos ensues outside Garden, Spurs beat Knicks in Game 3

US President Donald Trump was loudly booed when he was shown on the big screen during the NBA Finals.

Mr Trump's appearance at the game caused havoc outside of Madison Square Garden, with long lines to get in.

The San Antonio Spurs defeated the New York Knicks, with Game 4 of the series to be played on Thursday (AEST). The Knicks lead the best-of-seven series 2-1.

US President Donald Trump was booed loudly by fans inside Madison Square Garden when he was shown on video screens during the national anthem before Game 3 of the NBA Finals this morning (AEST).

Mr Trump was shown for several seconds giving a military salute. The boos ended when the US flag followed him on the screens and fans cheered when New York Knicks players were shown.

Mr Trump was watching from Knicks owner James Dolan's suite, along with granddaughter Kai, personal adviser Boris Epshteyn, and cabinet secretaries Lee Zeldin, Sean Duffy and Doug Burgum. He is the first sitting president to attend an NBA Finals game.

Mr Trump's Marine One helicopter flew from his home in New Jersey and landed near Wall Street before his motorcade made its way up through Manhattan and to the arena about an hour before tip-off.

He encountered a handful of people making rude gestures, and outside the area, one group held signs saying, "Trump must go".

During the afternoon before Mr Trump's arrival, the New York Police Department and the US Secret Service set up a large perimeter surrounding Madison Square Garden.

Fans lined up to get inside the arena more than four hours before tip-off, in a scene more closely resembling New Year's Eve in Times Square than the usual lead-up to a basketball game.

Knicks fans flip the bird at US President Donald Trump during a watch party in New York's Bryant Park. (Getty Images: Adam Gray)

They were required to provide a ticket or pass to get past various checkpoints, along with going through a Transportation Security Administration-style magnetometer.

Secret Service personnel and police were positioned at every corner and in large numbers. Daily commuters, tourists visiting Manhattan and fans were all confounded at various times as they tried to move through security.

After travelling from his new home in Florida for the game, Knicks fan Greg Weldon said the main inconvenience was the lack of information.

"We've asked so many cops, secret service, guys with machine guns, what to do, where should we go," he said.

New York Knicks fans wait to enter Madison Square Garden ahead of the game. (Getty Images: Craig T Fruchtman)

Knicks coach Mike Brown and Spurs counte