WASHINGTON — Speaking softly, nervously and in detail, Brian McNamee testified about the life-changing moment when, he said, he first gave Roger Clemens a “booty shot” of steroids.

The government’s star witness in the Clemens perjury retrial took the stand Monday and told the jury that he injected one of baseball’s most successful pitchers with steroids about eight to 10 times when they were with the Toronto Blue Jays in 1998.

“I knew what I was doing was illegal,” McNamee said. “I wish to God I could take it back.”

Clemens is charged with lying to Congress when he testified in 2008 that he had never used steroids or human growth hormone. The first attempt to try him last July ended in a mistrial when prosecutors showed the jury a snippet of videotaped evidence that had been ruled inadmissible.

The retrial took until its fifth week to get to the heart of the government’s case: McNamee is the only person who will claim firsthand knowledge of Clemens using performance-enhancing drugs.

In his thick New York accent, McNamee covered a lot of ground in about four hours on the stand — and he still has much more to tell when he returns today. He recalled how he met Clemens when McNamee was the strength and conditioning coach of the Blue Jays during the 1998 season. He said Clemens gave him a $1,000 tip at the end of spring training, that Clemens approached him one day in the clubhouse and asked him to get rid of a bag of some 20 to 30 bottles of steroids.

Then came the fateful day in June when he was asked by Clemens to come to Clemens’ apartment in the Blue Jays’ Skydome stadium after a game.

McNamee said he found alcohol, needle and gauze and the anabolic steroid Winstrol laid out in the bathroom. He said he felt “a little uncomfortable” while preparing the shot because he’d never done anything like it before. He said he then walked into Clemens’ bedroom.

“Roger pulled down his pants, exposing his right buttocks cheek to me,” McNamee said. A few seconds later, Clemens said he was ready. McNamee said he then “plunged the fluid in into his buttocks.”

After it was done, they “exchanged pleasantries,” according to McNamee.

“That,” McNamee concluded softly, “was the first time I injected Roger Clemens.”