MONACO -- Formula One championship leader Sebastian Vettel secured his fifth pole of the season after posting the fastest time in Saturday's crash-marred qualifying session at the Monaco Grand Prix.
Vettel is already threatening to run away with the championship after earning four wins in five races to build a commanding lead over Lewis Hamilton. The German can add to his advantage with his first victory in Monaco.
"It's a bit of a casino, a lot of things going on," Vettel said. "It's a long race, 78 laps. Pole position here is very important so we can be very happy with that, although there's no guarantees for tomorrow."
"If you'd asked me before qualifying if I would have taken third, I probably would have," Webber said. "Absolutely it is possible to win (from third on the grid). Qualifying here is crucial, no question about that.
"Pole is certainly nice to have. Seb did a great lap again today and deserves the pole, JB did a great run as well."
Webber took the pole at last week's Spanish Grand Prix and Vettel captured the previous four poles. The 23-year-old German leads McLaren's Hamilton by 41 points in the standings.
"I had to wait a long time to get the pole, but I'm very pleased," said Vettel, F1's youngest ever champion.
It was a disappointing session for Ferrari's Fernando Alonso, who was quickest in the final practice on Saturday morning, but dropped to fourth in qualifying.
Mexico's Sergio Perez was likely to miss Sunday's race after sustaining a concussion and thigh injury during a violent crash near the end of qualifying. Perez was taken to Princess Grace hospital and was expected to be kept overnight.
"It's difficult to keep the focus and it's difficult to keep the concentration when you don't know what's going on," Vettel said. "We all wish him the best."
Perez was the third driver to crash Saturday. The accident severely hampered Hamilton, as he had yet to set a lapped time and the heat went out of his tires as he waited nearly 40 minutes for qualifying to resume.
Motor sport's governing body, the FIA, said the 2008 F1 champion will start Sunday's race in from ninth on the grid rather than seventh, because his time from the third qualifying session was nullified for cutting a chicane.
That moves Mercedes driver Nico Rosberg up to seventh in his Mercedes.
The English driver was clearly disappointed after being fastest in the two opening qualifying sessions.
"I felt I'd been driving well all weekend and I had the pace to be on pole. I'm certain of that," Hamilton said. "I'll do my best from my grid slot and, while it'll be tough to win from there, I won't give up."
Perez, racing in the third session for the first time this season, lost control of his car as he was about to come out of the tunnel. The front slid to the right and slammed into the tunnel barrier, spinning the car round and sending it into a tire wall on the circuit.
Button had a similar tunnel crash in 2003.
"I know how much it hurts, so hopefully he'll be OK," Button said.
A medical crew rushed to the car, and a giant blanket was immediately draped over it. After a few minutes Perez was taken away for treatment and the battered car was lifted off the track.
"It is with great relief that the Sauber F1 team received the news that Sergio Perez has no serious injuries," the team said in a statement. "The doctors said Perez had suffered concussion and a sprained thigh, but no broken bones and, following a scan, they could find no further injuries."
Alonso had the fastest time in the third and final practice session in the morning, after posting the quickest time in Thursday's second practice and raising hopes Ferrari could get its first win of the season in Monaco.
Ferrari has been lagging behind Red Bull, but this weekend was the first time it has been faster in practice.
Alonso, last year's F1 runner-up, already trails Vettel by 67 points and needs a good performance from the second row Sunday to put any kind of pressure on Vettel. Alonso's best place this season was third in Turkey.
"Fernando looked competitive (in practice), I don't know what happened to them (Ferrari) in qualifying," Vettel said.
In a disappointing afternoon for Renault, both Vitaly Petrov and Nick Heidfeld missed out on making the third qualifying stint, suggesting a problem with downforce as their cars were bumping around the track.
Practice was interrupted twice in the morning as drivers raced around the sinewy, barrier-lined circuit under perfect blue skies. Rosberg and Hispania's Vitantonio Liuzzi were both unhurt after crashes.