COMPIEGNE, France — Fabian Cancellara, who opened the Tour de France on the Mall in London with victory in the prologue time trial, produced an unexpected end to the race’s longest stage yesterday, 148 miles from Waregem, Belgium, to Compiègne.

It is always unusual to see the race’s actual overall leader take the day’s honors, but Cancellara, his yellow leader’s jersey shining brightly, raced away from the front of a sprinting field to catch the day’s four breakaway riders in the last 400 yards to win stage three and increase his overall lead to 33 seconds.

It was a dramatic end to a long day of nearly seven hours as the fields of Picardy passed by at the slow average speed for much of the day of just 20 mph.

Two riders, Matthieu Ladagnous (Francaise des Jeux) and a former French champion, Nicolas Vogondy (Agritubel), rode up front often with a lead of 14 minutes, after breaking clear just three miles into the stage.

There is nothing unusual about this on such a long stage, but when the pair was joined by Stephane Auge (Cofidis) and Frederick Willems (Liquigas) with 30 miles remaining, alarm bells should have started ringing.

Auge was the biggest threat to David Millar’s lead in the King of the Mountains competition, but the Scot surrendered it without a fight and there was no chase by him or his Spanish team as the only climb of the day approached.

Auge won it, and pulled on Millar’s jersey as the new leader last night.

Millar, however, did hold on to third place overall, 41 seconds behind Cancellara, while Britain’s Bradley Wiggins also retained his fifth place overall, with the Alps to come this weekend.

The four leading riders realized that with a lead still in excess of three minutes, and with only 20 miles to go to Compiègne, they had a chance to beat the sprinters home, so they lifted the speed to a much more creditable 35 mph.

The teams of the sprinters joined forces to chase and, despite a tumble by yesterday’s winner, Gert Steegmans, on a narrow river bridge, the gap fell to just 30 seconds with three miles to go. But still the quarter never gave up on holding on to a share of the day’s prizes.

Then, as the field turned right on to a short, cobbled climb toward the finish, Cancellara took his chance and, showing enormous strength, he raced away from the field to catch and pass the four hapless leaders in sight of the line.

Shaking his head in disbelief, Cancellara crossed the line with both arms raised as all of the sprinters, led home by Germany’s Erik Zabel, aired their frustration at having been beaten by a world time trial champion who rarely wins a road race.

Britain’s Mark Cavendish, at the third attempt, finally reached the finishing line in the main pack, demonstrating how good he could be one day as the Isle of Man rider took ninth place in his first shot at a Tour de France stage win.

For two days, Cavendish has trailed home with cuts and bruises after crashing near the end of a stage, but yesterday the youngster said: “I’ve got grazes and I’ve fallen twice, but I love the race.”

At least his confidence remains intact after a hard baptism in the three-week event.