It was just over three seconds from Genzebe Dibaba's world record of 3.50.07 from 2015 and Hassan clapped his hands in delight after the race.
Her personal best is 3.51.95.
Despite the fine victory time, Hassan was threatened throughout the riot.
Faith Kipyegon looked to be able to pass at the beginning of the race but finished second at 3.53.91, which was a new Kenyan record.
Laura Muir was third in the high-class race at 3.55.59.
The first seven set some kind of annual best or record.