Nico Rosberg heads Mark Webber in second practice

Mercedes driver Nico Rosberg was in impressive form in practice at the Bahrain Grand Prix - nearly half a second clear of the field.

The Mercedes driver, who was on pole by a similar margin in China last weekend before scoring his maiden victory, headed Red Bull's Mark Webber.

World champion Sebastian Vettel was next, from McLaren's Lewis Hamilton.

The second Mercedes of Michael Schumacher was fifth, ahead of Hamilton's team-mate Jenson Button.

But Hamilton, who set the pace in the first session in the morning, was nearly a second slower than Rosberg's fastest time of one minute 32.816 seconds.

Webber was 0.446secs behind the German, with team-mate Vettel a further 0.263secs adrift.

Webber suffered a failure in his KERS power-boost system, which would have cost him about 0.3secs a lap in ultimate pace.

The session was overshadowed by ongoing concerns about the wisdom of racing in Bahrain this weekend amid continuing civil unrest.

Force India chose not to run in the second session so their team members could return to their hotel before dark.

That decision came after four mechanics were briefly caught in a clash between protestors with petrol bombs and police firing tear gas on their way back from the track on Wednesday.

And on Friday it emerged that a group of Sauber mechanics also encountered trouble on their way back to the capital Manama on Thursday.

The crown prince of Bahrain addressed the media immediately after the second practice session.

He said: "We are not trying to say we are perfect. We are a real country with real issues. I genuinely believe this race is a force for good. It unites many people from many different backgrounds under the roof of Formula 1."

F1 boss Bernie Ecclestone has told BBC Sport that it was "absolutely 100%" the right decision to race in Bahrain.

He had said at the Chinese Grand Prix last weekend that he would be "surprised" if anyone from F1 encountered any trouble in the Gulf state.

McLaren appeared to be struggling in the second session. Hamilton, the championship leader after finishing third in the first three races of the season, told his engineers the team would have to "go back to the settings we used this morning".

And Button, who was 0.499secs slower than Hamilton, added over the team radio: "I'm feeling massively no grip. Understeer at high speed and then oversteer on corner exit."

Ferrari were even further behind. Fernando Alonso, third in the championship thanks largely to his unexpected victory in the rain in Malaysia, was eighth fastest and 1.633 seconds off the pace.

The Spaniard had trouble making the softer of the two tyre options last more than one lap.

Schumacher may have been closer to Rosberg but he did not manage to get a clean lap when he ran the softer of the two tyres.

BBC 5 live analyst Jaime Alguersuari said: "Michael Schumacher tried one flying lap on the soft tyres, but hit traffic. Then he tried it again and was one second slower than his team-mate.

"It just shows that if you don't do your fastest lap time on your first run you don't get the time. The tyre gets so hot that you can't do it again."