"It's Monaco. You never get ahead of yourself here because you get bitten in the arse," he added.
The top five were split by just 4.1 seconds, running nose-to-tail through the hairpin, with Webber's team mate and double world champion Sebastian Vettel fourth ahead of McLaren's Lewis Hamilton.
Alonso has 76 points with Vettel and Webber both on 73.
In Image: Red Bull driver Mark Webber of Australia, leads Mercedes Grand Prix driver Nico Rosberg of Germany, during the Monaco F1 Grand Prix