Asian man keeps cool during intense racial attack

An Asian man's reasoned response to a disturbing racial attack has been caught on camera.

The victim, who was on a train in San Francisco, was subjected to slaps and offensive slurs by an older white man on his journey home.

The entire incident was filmed by another passenger and later shared on social media site Reddit, where it went viral.

"N****r. I'll punch you down in your seat you n****r. You Chinese n****r," he taunted, throwing an open-palm slap at his face. "You think I'm afraid of you?"

"I never said that, sir," the victim retorted, smiling up at his attacker.

The racist man, taken aback by his victim's calm reply, then slapped him again before adding, "I hate you f**king Chinese f**ks".

That riled up the Asian man for the first time, and he stood up and shouted a threat: "You put your hands on me one more time, you mother**ker."

However he was calmed by another passenger, and the incident was later reported to Bay Area Rapid Transit Police.

An investigation has been launched, and authorities are trying to identify and locate the attacker.