By Brian Homewood

HOCKENHEIM, Germany (Reuters) - Formula One championship leader Fernando Alonso was fastest in the final practice session for the German Grand Prix on Saturday.

The session began in sunshine and ended in a torrential downpour as the rain which hampered Friday's two practices returned to the Hockenheim circuit.

Lewis Hamilton, who will be taking part in his 100th grand prix on Sunday, was second, giving some encouragement to his McLaren team after their disappointing last two outings.

However, his team mate Jenson Button was slowest of the 24 drivers.

Mexico's Sergio Perez was third for Sauber while Red Bull's double world champion Sebastian Vettel, who is attempting to win a race in his native Germany for the first time, was fifth.

Vettel, who led for part of the session, was one place behind his Red Bull team mate Mark Webber, second in the championship.

Webber will be penalized five places on the starting grid for an unscheduled gearbox change along with Nico Rosberg and Romain Grosjean.

Alonso, last year's German Grand Prix winner at Nuerburgring, set a time of one minute 16.014 seconds for Ferrari just after the halfway point of the session.

The deteriorating weather prevented the usual spate of improved times late in the session.

Seven-times world champions Michael Schumacher, who had an embarrassing crash late in Friday's second session, struggled and managed only 15th for Mercedes.

(Reporting By Brian Homewood; Editing by Alastair Himmer)