GOTEBORG, Sweden (AP) _ Dennis Mitchell's bid to dethrone Linford Christie as world champion ended after less than 50 meters of his first race today when the American pulled up injured in his heat at the World Championships.

Bronze medalist behind Christie at the Worlds at Stuttgart two years ago and also third behind the British runner at the Barcelona Olympics in 1992, Mitchell was considered one of the leading contenders.

Mitchell, the fourth fastest 100 meter runner of all time at 9.91, was at full speed when he grimaced and he pulled up with cramps to the left quad muscle.

He said he suffered mild cramps during early practice but didn't think the problem would get worse.

``It was nothing to be concerned about but it escalated into something I could not handle,'' the American said.

While Cuba's Jose Isasi ran on to win the heat in a modest 10.44 and qualify for the next round, Mitchell was helped off the track by officials.

Christie, who has said he will retire at the end of this season, looked confident as he won his first two rounds but was well beaten on times.

The defending titlist clocked 10.26 in round one and improved to 10.15 in the second.

Mike Marsh, who won the U.S. trials, was fastest qualifier for Sunday's semifinal although his 10.03 was wind-aided. But he gained an edge over one of the other title contenders, Namibia's Frankie Fredericks, who clocked 10.09 for second in the same second round heat.

Ato Boldon, a sprinter from Trinidad and Tobago who is coached at UCLA by respected American sprint coach John Smith, ran a best ever 10.04 into a slight wind and will be considered a contender for the title, when that is decided Sunday.

He scored a notable victory over Canada's Bruny Surin, 10.14, with American sprinter Maurice Greene back in sixth place in 10.35 and out of the competition.

Canada's Donovan Bailey, who has run the fastest time this season, 9.91 at Montreal three weeks ago, clocked 10.18 to win his heat and gain the semifinal.

Michael Johnson, meanwhile, made a confident start to his ambitious bid to win an unprecedented double. But he eased up so much in his heat he was only fourth fastest in the first round of the 400 meters.

The defending champion, who also won the 200-meter title in 1991, clocked 45.49 to win his heat. He opened up a huge lead in the middle 200 of the race, but, by the finish line the American was almost at walking pace with Abednego Matilu of Kenya well back in 45.80 and Jamaica's Davian Clarke third in 45.92.

``I went fairly good,'' Johnson said. ``I planned to go out fast for the first 200 meters but I decided to take it out for 300. I plan to get through the rounds as relaxed as I can. I felt good.

``Now I have the first round out of the way I feel confident, I knew I was coming in in good shape,'' Johnson said. ``I'm just trying to get through the early rounds as easy as possible.''

Johnson hopes to go for the same 200-400 double at next summer's Olympics in Atlanta and heard some encouraging news today when IAAF officials said they would look into the possibility of altering the schedule so he could compete in both.

World record holder Butch Reynolds also looked in form as he won his 400m heat from the outside lane in 45.60 with no one putting him under any pressure.

But the fastest of the first round runners turned out to be Ibrahim Ismail of Qatar, who clocked 45.10 to win his heat, ahead of David Kamoga of Uganda and the third American in the field, Darnell Hall. Kamoga and Hall both had 45.34 and all three runners were faster than Johnson.

The 32 qualifiers from the first round, go into a second round, followed by a semifinal and the final is on Wednesday.

The only two finals on the opening day of action on the track were the women's marathon and the women's shot put.