After Germaine Mason had claimed Britain's first athletics medal of the Games with a surprise silver in the high jump, Ohuruogu produced a storming finish to overhaul favourite Sanya Richards and add Olympic gold to the world title she won in Osaka last year.

Ohuruogu won in a time of 49.62 seconds as Richards blamed a hamstring injury for tying up badly down the home straight and could only finish third behind Jamaica's Shericka Williams.

"I don't know where I am right now," said Ohuruogu. "You never think it's a reality, it's something you dream about.

"As I came across the line I thought 'Oh my gosh' – I don't know what to say.

"I'm just so proud of myself. Today I warmed up for an hour and a half. I had to keep stopping and sitting down to sort my head out.

"I felt tired, I hadn't slept for the last two nights, the pressure was getting to me.

"I thought 'If I don't win, what will happen?' As it gets closer and closer you get more scared and realise it is a lot harder than you think it is."

Just over two years ago Ohuruogu was considering quitting athletics after being suspended for 12 months for missing three out-of-competition drugs tests.

But she continued to train during her ban and returned to competition to win the World Championships in Japan last year ahead of team-mate Nicola Sanders, less than a month after the suspension expired.

Her place on the British team was only secured after she successfully appealed against a BOA ban from competing at the Olympics.

"I believe I have been given an amazing talent and I am thankful I am able to come out and perform because championships is where it's at," added the 24-year-old, who also won Commonwealth gold in Melbourne in 2006.

"You have to believe in yourself, you believe in the talent you have been blessed with and that you can go out and do it.

"I just know I am here and have won a gold medal and that is all that matters right now.

"I don't really care what people think or say, they can say what they like. I have come here and got what I wanted and I'm happy."

Richards was not in Osaka last year after failing to qualify but looked the woman to beat after the heats.

However, the American went off too quickly in the final and paid the price.

"I had a race plan but as usual it went out the window," added Ohuruogu. "I was in my own world. People said 'Did you know Sanya had gone off very hard?' but she was in lane seven and I couldn't see her.

"All I had to work off was the Russian girl outside me who I knew would go off very hard and at 200m I was annoyed with myself that I had left myself too much work."

While Ohuruogu's performance was not a surprise, Mason's silver was a welcome bonus.

The 25-year-old, who was born in Jamaica but switched nationalities in 2006, celebrated Britain's first high jump medal since Steve Smith won bronze in Atlanta in 1996. Before Smith, brothers Cornelius and Patrick Leahy in 1908 and 1900 respectively had been Britain's only male Olympic medallists.

Britain could now end the Beijing Olympics with an astonishing 20 golds – and whatever happens look certain to beat Australia in the medal table.

Ohuruogu's victory secured Britain's 16th gold of the Games so far and Team GB are now five ahead of Australia.

Ohuruogu's success, added to two cycling golds by Chris Hoy and Victoria Pendleton and one by Yorkshire sailor Paul Goodison, saw Britain reinforce their third place in the table behind leaders China and the USA.

British Olympic Association chef de mission Simon Clegg said: "There have continued to be some absolutely brilliant performances by Team GB athletes across many different sports.

"It has been another outstanding day and a great gold medal on the track by Christine who ran a fantastic race."

Asked whether Britain could reach the 20-gold mark, Clegg added: "Let's just say we have a number of people still to compete who are quite capable of winning gold and we continue to go from strength to strength."

Phillips Idowu is favourite to win the men's triple jump tomorrow and Shanaze Read the women's BMX cycling, while Tim Brabants is a front-runner in flatwater kayaking.

There are also possible gold medal chances for Iain Percy and Andrew Simpson in sailing's Star class and Nick Dempsey in the RS:X windsurfer category.

Australia have only five good gold medal chances remaining but Clegg believes the Russians could yet steal third place from Team GB as they are still favourites for a number of disciplines.