Iron Man 3 has stormed to the top of the US box office, with the second biggest ever opening weekend in the US. The comic book sequel, starring Robert Downey Jr, took an estimated $175.3m (£112.5m) across the weekend. Only 2012's The Avengers, which united Iron Man with fellow Marvel characters, had a bigger opening, taking $207.4m (£133.1m) when it opened last May. Iron Man 3 is the final episode in the trilogy, and follows Tony Stark's battle with terrorist "the Mandarin". Directed by Shane Black, who previously worked with Downey Jr on Kiss Kiss Bang Bang, the film co-stars Gwyneth Paltrow, as Pepper Potts, and Sir Ben Kingsley as the Mandarin. The third film, which has enjoyed largely positive reviews. has easily outstripped the opening weekends of the first two films, which took $98.6m and $128.1m respectively. Kenneth Turan, writing in The Los Angeles Times, called it "unexpectedly - and successfully - darker and more serious than its predecessors", while CNN's Tom Charity called it "a confidently tongue-in-cheek piece of blockbuster engineering". The film opened a week earlier overseas, taking $504.8m (£324.1m) at the international box office - meaning it has already grossed $680.1m (£436.7m) in total. Downey Jr is expected to reunite with fellow comic book characters Captain America, Thor and the Hulk in the forthcoming Avengers sequel, which is scheduled to be released in 2015.