PHOTOS: US breaks 27-year-old record in women's 4x100

Pointing the baton at the clock showing the world-record time of 40.82 seconds, Carmelita Jeter anchored the United States on Friday night to its first Olympic gold medal in the women's 4x100-metre relay since 1996.

Tianna Madison, 200-meter champion Allyson Felix and Bianca Knight gave the U.S. a big lead heading into the anchor leg by Jeter, who already owned a silver medal from the 100 and a bronze from the 200 in London.

Their final time cut more than a half-second off the old record of 41.37 run by East Germany in 1985.

Jamaica won the silver medal Friday in a national record of 41.41 seconds, with 100 champion Shelly-Ann Fraser-Pryce, Sherone Simpson, Veronica Campbell-Brown and Kerron Stewart bringing the baton around.

The bronze went to the Ukraine in 42.04.

With Knight approaching for the final handoff, Jeter took nine strides, reached her hand back and took a perfect exchange.

Jeter was staring at the clock as she covered the final 10 metres and used the stick in her left hand to make sure everyone else also had their eyes trained on the bright orange numbers on the trackside clock.

Afterward, the quartet of champions paused to watch a replay of their record performance on the scoreboard at 80,000-seat Olympic Stadium. When Jeter was shown crossing the finish line, Knight punched the air.

The perfect trip around the track ended a string of disappointments for the U.S. in the event. At Beijing four years ago, the Americans didn't even reach the final because Torri Edwards and Lauryn Williams bobbled the last exchange in the semi-finals.

That marked the first time since 1948 that the U.S. wasn't involved in the women's 4x100 medal race at the Summer Games.

Meseret Defar upstaged fellow Ethiopian Tirunesh Dibaba to win the Olympic 5,000 metres Friday night.

Defar, the 2004 Olympic champion, overtook Dibaba in the final stretch and held on to win the gold medal in 15 minutes, 4.25 seconds.

World champion Vivian Cheruiyot of Kenya surged into second place in the last 50 metres to claim the silver medal in 15.04.73. Dibaba finished with the bronze in 15:05.15.

Dibaba was aiming to repeat as double Olympic champion in the 5,000 and 10,000 meters. She won the distance double in Beijing in 2008 and retained her 10,000 title on the first night of the track and field competition at the London Games.

Asli Cakir Alptekin led a Turkish 1-2 when she held off Gamze Bulut to win the women's Olympic 1,500 metres gold on Friday after delivering a powerful final lap in a tactical, bunched race.

Alptekin, who won the European title in Helsinki two months ago, served a two-year ban for doping after testing positive in 2004.

She looked powerful and strong on Friday, coming home in four minutes, 10.23 with Bulut second in 4:10.40.

Maryam Yusuf Jamal of Bahrain was third and American Morgan Uceny, who fell in the 2011 world championship final, tumbled again on Friday and beat the track in tearful frustration.

For the fourth time in five Olympics Britain were disqualified from the men's 4x100 metres relay on Friday, robbing the 2004 winners of a place in Sunday's final, the last event of the track programme when they would have been real medal contenders.

Defending champions Jamaica, without the rested Usain Bolt, advanced with a blistering time as did the United States, to set up another mouth-watering showdown when Bolt will return hoping to complete his "double-treble" having bagged the 100m and 200m.

The United States, anchored by 100m bronze medallist Justin Gatlin, won their semi in 37.38 and Jamaica, with 100 and 200 runner-up Yohan Blake running the third leg took theirs in 37.39, the third and fourth-fastest times ever recorded.

Tatyana Lysenko of Russia set an Olympic record to win the hammer throw gold medal on Friday night.

Lysenko, the former world-record holder who served a two-year doping ban until 2009, set the games record with her first attempt of 77.56 metres and then improved it with her fifth at 78.18. Aksana Miankova set the old mark of 76.34 at Beijing.

Anita Wlodarczyk of Poland earned the silver at 77.60 with her last throw and China's Zhang Wenxiu took bronze at 76.34.

Miankova, the 2008 champion, placed sixth.

Bahamas edged out favourites the United States to take gold in the Olympic men's 4x400 metres relay final on Friday.

Bahamas' Chris Brown, Demetrius Pinder, Michael Mathieu and Ramon Miller sped home in a time of 2 minutes 56.72 seconds to take gold.

Bryshon Nellum, Joshua Mance, Tony McQuay and Angelo Taylor claimed silver for the U.S. with a time of 2:57.05, ending a run of seven successive golds in the event.

Trinidad and Tobago won the bronze with hosts Britain fourth.