Four national flames, kindled last week, were united in a cauldron at a ceremony in Stoke Mandeville - the spiritual home of the Paralympics.
A flame lit from that cauldron is being carried 92 miles from Buckinghamshire to London's Olympic Stadium.
The Queen and the Duke and Duchess of Cambridge are among those attending Wednesday's opening ceremony.
The Paralympic flame is being carried by some 580 torchbearers.

'Inspiring a generation'

About 3,000 invited guests, including Paralympians, representatives from disability groups and local residents, attended Tuesday evening's ceremony at Stoke Mandeville Stadium.
Some 150 local residents took part in a lantern procession and formed a guard of honour for eight torchbearers who carried flames representing England, Scotland, Northern Ireland and Wales.
The children who were invited to take part in the procession, together with their parents, had gathered at Stoke Mandeville on 19-20 August to make the lanterns out of canes, tissue paper and sticky tape.
One of those involved was 12-year-old William Lansdown from Hazlemere in Bucks, who has Down's Syndrome and attends a sports group for disabled children.
"The lanterns looked brilliant," said William's mother, Lynn. "It was a great atmosphere, with the emphasis on families taking part and not just disabled people.
"The fact that so many children were involved made it special, given the theme of inspiring a generation to do more sport."
Earlier, performers entertained the crowds ahead of speeches by International Paralympic Committee (IPC) president Sir Philip Craven, Lord Coe, chairman of Games organisers Locog, and Culture Secretary Jeremy Hunt.
Eva Loeffler, the daughter of the founder of the Paralympic Games, Dr Ludwig Guttman, paid tribute to the role the Stoke Mandeville Games - and her father - had in defining the modern Paralympic movement.
Lord Coe addressed the crowd, saying he was "excited" to be at the home of the Games on the eve of their opening.
Speaking of Dr Guttman, he said: "It is simply not possible to stand here without feeling a mountainous debt of gratitude for one of the world's great visionaries."
Carrying the English flame was Katie Piper and Paralympian Tony Griffin.
During a 10-year career Mr Griffin won 38 medals and works as Bolton's Sports Ambassador promoting disabled sport.
The Scottish flame was carried by boxer Jon Jo Look, who has a prosthetic leg and coaches youngsters in the sport, and Noel McShane, who set up the National Wheelchair Tennis Association of Great Britain and the British Open Wheelchair Tennis Championships.
Darren Ferguson, a special constable who talked down a distressed man from a bridge, and Joseph Morris, who saved a girl from drowning in a river, carried the Northern Ireland flame.
Julie Gilbert and Marsha Wiseman carried the Welsh flame.

'Magical welcome'

Shortly after 20:00 BST, the first team of torchbearers - Paralympians chosen by the IPC - left the stadium, signalling the start of the 24-hour torch relay.
Just before midnight the torch was carried through the village of Weston Turville, in Buckinghamshire, where residents lit candles to line the route.
A London 2012 spokeswoman described the welcome as "magical".
Making up the team were:

• IPC president Sir Philip Craven took part in five Paralympic Games mainly in wheelchair basketball, and swimming.
• Baroness Susan Masham represented GB at the first two Paralympic Games winning medals in swimming and table tennis.
• Caz Walton has been involved in every Paralympic Games since 1964 as both an athlete and team manager.
• Sally Haynes took part in the first Paralympic Games in Rome in 1960 and went on to compete at a further three Games winning medals in the Epee discipline of wheelchair fencing and table tennis.
• Jane Blackburn took part in five Paralympic Games between 1972 and 1992 competing in archery, athletics, lawn bowls, swimming and table tennis. and winning 11 Paralympic medals including five golds.

Highlights of the relay route include visits to the National Spinal Injuries Centre, Stoke Mandeville, Lord's Cricket Ground, London Zoo and the Abbey Road crossing made famous by the Beatles.
When it arrives at the Olympic Park in Stratford, east London, it will be used to light the cauldron during the opening ceremony of the Games.
The event, called Enlightenment and created by Bradley Hemmings and Jenny Sealey, will showcase the skills of disabled artists with a cast of 3,000 adult volunteers including injured soldiers and past Paralympic athletes.
The four national flames were kindled at the summit of the highest peaks in Scotland, Northern Ireland, England and Wales last week.
They were used to light ceremonial cauldrons in London's Trafalgar Square on Friday, outside Stormont in Northern Ireland on Saturday, at the Mound in Edinburgh on Sunday and outside City Hall in Cardiff on Monday.