LONDON - Britain's Met Office has forecast a mix of sunshine and showers in London on Friday, when hundreds of thousands of people are expected to line the streets for the royal wedding of Prince William and Kate Middleton.

Chief Forecaster Andy Page said the day will start cloudy with an increased possibility of rain about midday, just around the time the couple are due to be riding in an open-topped carriage past cheering crowds from Westminster Abbey to Buckingham Palace after the wedding service.

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"After a cloudy start to the day, with a 10 percent chance of a shower, we should see some sunny spells developing over London during the morning," Page said on the Met Office website.

"The chance of a shower increases to 30 percent for the London area around lunchtime as temperatures reach 17 Celsius".

The wedding service begins at 11 am local time (1000 GMT) and the royal couple will leave the abbey 75 minutes later.

The Met Office website also advised those attending some 5,500 street parties around Britain that heavy showers might break out in the afternoon.

The site will be updated on Friday.

Most of the key events around the royal wedding in London on Friday take place between 0715 and 1230 GMT, although it could take several more hours for crowds to disperse.

Middleton and William may take heart from the mixed weather outlook for their big day, however -- tradition has it that rain on the wedding day is a good omen.