Roberto Duran, often smiling contemptuously at his opponent during the last five rounds, mixed strength and boxing ability to score a 15-round unanimous decision over Sugar Ray Leonard and win the World Boxing Council welterweight championship last night in Montreal.

Duran set the tempo from the start. He negated Leonard's hand and foot speed by feinting with his head and shoulders, then bulling the defending champion into the ropes.

In gaining the title on a cool, damp night at Olympic Stadium, the 29-year-old Panamanian became only the third man to win both the lightweight and welterweight championships. He held the lighweight crown from 1972 until last year, when he surrendered it to move up into the welterweight class.

Judge Harry Dibbs of Great Britain scored the fight for Duran, 145-144, and judge Ramon Baldeyrou of France scored it for the new champion, 146-144. Judge Angelo Poletti of Italy scored the fight for Duran, 148-147.

It was the first defeat in 28 professional fights for Leonard, 24, who grew up and still lives in Prince George's County. Duran scored his 70th career victory, including 55 knockouts. He has lost only once in a 13-year professional career.

The fight was supposed to feature Leonard's fast hands. But Duran showed the quickest hands throughout the bout. He fought a consistent fight while Leonard, unable to handle Duran's bullish tactics, was content to fight only in flurries.

Only in the final rounds, when he knew he needed a knockout, did Leonard show any of the flash that had brought him 18 knockout victories.

Duran came out for the last round looking like a fighter who knew he had won. With about 2,000 Panamanians waving tiny flags and pushing themselves to the point of frenzy, Duran again bulled into Leonard. As the round drew to a close, he danced away from Leonard and across the ring. At the bell, he smiled and threw his arms into the air.

Duran's victory was not declared unanimous until well after the fight when judge Poletti's scorecard was retotaled and found to favor Duran, 148-147. That total had been announced in the ring as a 147-147 draw.