Anna Ford projected an eye test

The event marks the start of the first ever World Sight Day, which aims to raise awareness of blindness and the conditions associated with it.

Similar stunts in Cape Town, Hong Kong, Sao Paulo and Sydney will help raise international awareness of the steps that can be taken to avoid blindness.

Events elsewhere will include cataract and glaucoma screenings, sight tests and the distribution of free glasses.

Onlookers got a preview of the eye chart earlier in the evening when BBC newsreader Anna Ford projected it onto London's Moorfields Eye Hospital.

Regular tests

In the UK the focus will be on the links between diabetes, glaucoma and blindness.

The British Diabetic Association (BDA) and the International Glaucoma Association will stress the importance of regular eye tests.

David Wright, of the International Glaucoma Association, said that half a million people had the condition.

"The trouble is that of those half million, 125,000 have the condition and are completely unaware of that.

"Those are the people we are trying to reach and encourage to have a sight test."

Major cause

Diabetes is thought to be the biggest single cause of blindness in the UK because sufferers are more likely to get glaucoma.

World Sight Day started at Moorfields Hospital

That condition is caused by an increase in the pressure of the fluid inside the eye which can then damage the optic nerve.

Sufferers find that their vision may become patchy or they may lose the sight from the sides of their eyes leading to tunnel vision.

However, blindness can be avoided if glaucoma is detected early on.

Susan Lucas, director of care at the BDA said: "People still do not know, who have diabetes or other conditions, how important it is to have regular screening and they go blind - unnecessarily."