ANAHEIM, Calif. — Brock Lesnar was downright stunned when Cain Velasquez took his biggest shots, escaped his violent takedowns and just kept coming at him at UFC 121.

The baddest man in mixed martial arts was locked in the cage with an opponent who was just as tough, even more skilled — and totally unintimidated by his bad-boy aura.

Four minutes later, Velasquez also was the UFC heavyweight champion.

Velasquez stopped Lesnar late in the first round with a relentless flurry of punches Saturday night, claiming the title belt from the UFC’s biggest star.

“It’s a great feeling to have the belt,” the former Arizona State wrestler said calmly, looking down at the gaudy ornament on the podium before him. “Now the hard work comes in, definitely.”

Velasquez remained unbeaten and claimed MMA’s highest-profile belt by beating the fearsome Lesnar at everything he does best. The UFC’s top pay-per-view draw and champion for the past 23 months was largely helpless against Velasquez, who reduced him to a cowering defensive posture for the second half of their brief fight.

Only a slight case of nerves even shook Velasquez at all. After a frenetic opening minute featuring huge blows by both fighters, Velasquez battled back from two takedowns and never stopped pursuing the bigger champion.

“I felt great about the fight,” Velasquez said. “We knew his game plan going in, and it kind of did surprise me how hard he came forward. I froze. I wasn’t as relaxed as I should have been, but after that takedown he got on me, that’s when I was able to say, ‘Relax, relax.'”

Velasquez eventually staggered Lesnar across the octagon, with Lesnar stumbling to the canvas several times. Lesnar (5-2) tried to cover up near the cage, but Velasquez mercilessly rained down blows on Lesnar and eventually broke his guard, forcing referee Herb Dean to stop the fight with 48 seconds left in the round.

“He looked incredible tonight,” said UFC President Dana White, who didn’t seem disappointed to see his top attraction’s demise. “The guy gets better every time he fights. He’s incredibly well-rounded. I think he answered all the questions tonight.”

While Velasquez (9-0) scarcely made a mistake in the entire fight, Lesnar was left cut and seriously bloodied from Velasquez’s punches. Several ringside observers thought they heard Lesnar verbally ask for the fight to be stopped, although Velasquez said he was too busy punching to hear it.

“What can I say? He was better than me tonight,” Lesnar said in the octagon.

Velasquez exposed Lesnar’s clear deficiencies in standup fighting, just as Shane Carwin did in his own shot at Lesnar in July. But Velasquez didn’t punch himself into exhaustion, as Carwin did in Las Vegas, instead being more judicious about his strikes.

“We just had to pick our shots,” Velasquez said. “I knew that the ref wasn’t going to stop it that early. He was covering up well. I wanted some punches to connect, some elbows to get in there, so I really took my time and thought about where to land them.”

Lesnar’s third title defense came just four months after his return to the octagon following a yearlong bout with diverticulitis, an intestinal malady that threatened his life and forced him to revamp his training and diet. The former professional wrestler and football player adapted without losing the sheer bulk that makes him the toughest physical matchup in the UFC.

But he had never faced a fighter with the athleticism and well-rounded skills of Velasquez, who trains at a famed kickboxing academy in San Jose. Velasquez also drew motivation from the chance to become the first Mexican-American heavyweight champion in a major promotion in either MMA or boxing.

The UFC returned to the Los Angeles area for the first time in a year with its most anticipated event of the fall. The main event didn’t disappoint — although many fans in the sold-out arena likely didn’t expect the result after one round of pyrotechnics in the main event.

Velasquez entered the cage to cheers from his Latino fans in Orange County, while Lesnar drew a mix of boos and cheers when the bearded fighter walked to the cage.

The first 30 seconds were nonstop action, with both fighters trading haymakers and knees. Lesnar took down Velasquez twice with a loud thump as both fighters hit the ground, but Velasquez eventually pushed Lesnar back against the cage, testing both fighters’ strength — and Velasquez held his own despite giving away 2 inches and roughly 30 pounds to Lesnar.

When the punching resumed, Velasquez landed most of the blows. Lesnar stayed in a right-handed stance, but might have had trouble seeing out of his left eye, eventually leading to his stumbles and tumbles against the cage.

Earlier, Jake Shields won a contentious split decision over Martin Kampmann in his UFC debut at Honda Center, extending his career winning streak to 15 fights. Matt Hamill beat Tito Ortiz by unanimous decision to keep the former light heavyweight champion winless in the past four years, and Diego Sanchez beat Paulo Thiago by unanimous decision.

Although White agreed with the decision, most fans booed when Shields’ hand was raised after a slow, largely uneventful fight with Kampmann. Shields (26-4-1) hasn’t lost an MMA fight since December 2004, going through eight promotions since that defeat.