Manny Pacquiao, the fighting Filipino congressman, tried to lay down the law Saturday against challenger Shane Mosley, a reluctant warrior in the desert who mostly refused to engage the WBO welterweight champion.

Pacquiao, the world's preeminent pound-for-pound pugilist, knocked down Mosley once but he was unable to stop the former champion while winning an easy unanimous decision. Judges Glenn Trowbridge (119-108), Dave Moretti (120-108 ) and Duane Ford (120-107) all had huge margins for Pacquiao.

Before a pro-Pacquiao sellout crowd at the MGM Grand Garden Arena, the goateed ring assassin floored Mosley in the third round by firing a right-left combo that was set up by a subtle feint. Mosley, bleary-eyed on the canvas, rose to his feet but was wobbly as he fended off the champion.

Mosley, when he wasn't cautiously retreating, tried to jab his way in, but rarely used a lead right hand, a prime punch against a southpaw. The earlier knockdown put Mosley on the defensive.

After the fifth round, Mosley's face was puffy and red from Pacquiao's consistent jab. "I just fought the best fighter in the world. He has exceptional power. I've never been hit like that before," Mosley said.

By the end of the seventh round, the crowd booed, wanting more exchanges between the fighters. Several times, their heads clashed during exchanges, leading to warnings from referee Kenny Bayless.

Pacquiao appeared to get knocked down in the 10th round, but a reply showed Mosley shoved the champion to the canvas.

After the fight, Pacquiao revealed he was struggling with leg cramps during the fight. "I couldn't move because my left leg got tight. It's a problem I have been having," he said. Added trainer Freddie Roach: "He came back in the fourth round with muscle cramps in his left leg. … It was a very gutty performance in my mind because of his leg."

The champion was gracious despite Mosley's unwillingness to engage. "I thought Shane did a great job," Pacquiao said.

Earlier in the day, a vehicle Pacquiao was riding in was struck by his camp's security vehicle as the champion was returning from a church service at Mandalay Bay Hotel and Casino, where the fighter is staying. Pacquiao was shaken but unharmed, according to his camp.

Pacquiao was making his first ring appearance in Las Vegas since 2009. Pacquiao's last two fights, in 2010, were held at Cowboys Stadium in Dallas. He has said he plans to retire in 2013.

In the meantime, Top Rank promoter Bob Arum will continue to try to arrange one of the biggest battles in boxing history: Pacquiao versus the undefeated Floyd Mayweather Jr., the slick boxer whose once-glowing career has been derailed by legal, financial and personal issues.

Pacquiao (53-3-2, 38 KOs) won his 147-pound title in 2009 when he knocked out four-time world champion Miguel Cotto. His remarkable ring history includes eight world titles in as many weight divisions. He has not lost since Erik Morales defeated him in a 12-round decision in 2005, winning 14 consecutive bouts. Pacquiao turned pro at 106 pounds in 1995.

Mosley (46-7-1, 39 KOs) came into the championship contest with a 15-5 record in world title bouts, including 12 knockouts. He is a four-time world champion in three different weight classes, including two victories against Oscar de la Hoya.

Mosley has not won since 2009 when he stopped Antonio Margarito in nine rounds in Los Angeles to capture one alphabet version of the welterweight crown. He dropped a 12-round decision to Mayweather in Las Vegas last May and his fight against Sergio Mora ended in a disputed draw four months later in Los Angeles.

On the undercard, Wilfredo Vazquez Jr. (20-1-1) of Puerto Rico lost his junior featherweight title when Jorge Arce (57-6-2) of Mexico stopped him the 12th round. Vasquez's corner threw a water bottle into the ring to halt the action as referee Joe Cortez watched the challenger mercilessly hammer the former champion.

Earlier, former world middleweight champion Kelly Pavlik (37-2, 32 KOs) of Youngstown, Ohio, scored a majority 12-round decision against Alfonso Lopez (21-1, 16 KOs) from Cut and Shoot, Texas.