By Justin Palmer

LONDON (Reuters) - Four years on from storming to Olympic gold in a seemingly effortless world record time, either Usain Bolt will again be hailed a great champion or a new 100 meters sprint king will be crowned in London on Sunday.

The Jamaican triple gold medalist from Beijing has had his sprint authority challenged with the emergence of younger compatriot Yohan Blake, who took his friend and training partner's world title in Daegu, South Korea last year when Bolt false started, and set the world's leading mark this year.

Bolt came into the Games with questions over his fitness but enjoyed nothing more than a light workout when cantering through his first round heat on Saturday despite making a sloppy start.

If all the leading protagonists safely negotiate semi-final passage, the final (2055) has the makings of one of the all-time greats with a fast London track offering hope of a new world record.

Bolt will line up in the second semi-final with American Ryan Bailey, the fastest man in the first-round heats.

Blake goes in the last semi along with American Tyson Gay, while 2004 Olympic champion Justin Gatlin and former world record holder Asafa Powell are in the opening heat.

The first two in each heat and the next two fastest advance.

The 100 final is proceeded by what should be an exciting 400 meters showdown when Britain's Beijing champion Christine Ohuruogu continues her rivalry with American Sanya Richards-Ross, the hot favorite for gold in 2008 when she claimed bronze.

Russian Antonina Krivoshapka is also a threat having set the world's leading time this year.

Golds will also be won in the men's 3000m steeplechase, men's hammer and women's triple jump.

There is no morning session of competition in the stadium but all eyes will be on twice London marathon winner Mary Keitany of Kenya who returns to the capital's streets to bid for women's marathon gold.

Compatriot and world champion Edna Kiplagat will line up as probably her chief rival but for all their depth of running talent, a Kenyan woman has never won Olympic gold in the marathon.

The route, which starts (1000) and finishes in the Mall, takes in notable landmarks including Buckingham Palace, Admiralty Arch, Birdcage Walk, St Paul's Cathedral, Leadenhall, Tower Hill and the Houses of Parliament.

(Editing by Ed Osmond)