Zack Snyder's Watchmen duly leapt to the top of the US box office this weekend, after a strategic retreat by the competition left the superhero movie with a clear run to the top.

The big-budget adaptation of the Alan Moore graphic novel took $55.7m (£40m) from Friday night onwards, the highest opening of the year so far and the third highest March opening of all time.

The figure is still thought to have disappointed bosses at Warner Bros, however, who had hoped that Watchmen would fare even better than Snyder's directorial debut 300, another graphic novel adaptation, which debuted with $70m on the way to taking more than $200m in the US alone. One reason posited for the lower income is Watchmen's running time. At two hours and 40 minutes, it meant that cinemas were unable to schedule as many showings as they might have wished.

Internationally, Watchmen also fared well enough, bringing in another $27.5m. It is almost certain to top the UK box office when figures are announced later this week.

Despite its R rating and its niche subject matter, other distributors were careful not to put their movies up against Watchmen; as a result no other film made its debut in the top 10 this week.

The rest of the US top five were, in descending order, comedy Madea Goes to Jail, with $8.8m in its third week, thriller Taken, which continues to perform well with $7.45m in its sixth, Danny Boyle's Oscar-winning Slumdog Millionaire, with another $6.9m in its 17th week of release, and another comedy, Paul Blart: Mall Cop, which took $4.2m in its eighth week.