LYON, France (Reuters) - The Formula One community, and the wider world of motorsport, reacted with shock and prayers on social network Twitter for retired seven-time F1 world champion Michael Schumacher to win his biggest battle.

The 44-year-old German was in "critical" condition with head injuries after an off-piste skiing accident in the French Alps resort of Meribel, his agent said on Sunday.

"If anyone can pull through, it's him," said Britain's triple Indy 500 winner Dario Franchitti, who is still walking on crutches after a huge crash in October that ended his racing career.

"Come on Michael, give us one of those race stints at pure qualifying pace to win through, like you used to. You can do it," said Schumacher's former Benetton team-mate Martin Brundle.

Former Ferrari team-mate Felipe Massa, who suffered a near fatal head injury at the 2009 Hungarian Grand Prix, also reacted to the accident.

"I am praying for God to protect you my brother!! I wish you a speedy recovery Michael," wrote the Brazilian.

Schumacher is the most successful Formula One driver of all time with a record 91 race victories in an extraordinary career spanning more than two decades.