Megadeth main man Dave Mustaine at Sonisphere 2011 in the grounds of Knebworth House

ROCK festival Sonisphere 2011 has begun, with more than 50,000 metal fans packed into the grounds of Knebworth House for the three-day music event.

The crowd at Knebworth waiting for Slayer to come on stage

Gates opened to Knebworth for early campers yesterday (Thursday) and the music kicked off at 3pm today (Friday).

British heavy metal act Diamond Head were first up on the main Apollo stage and the Midlanders showed exactly why they’ve been such an influence on Friday night headliners Metallica.

They rounded off their 30-minute set with Am I Evil? – the song the Big 4 jammed to last summer in Europe.

Anthrax were the first of thrash metal’s Big 4 to take to the Apollo stage, minus founder member Scott Ian.

Megadeth and Slayer have also now performed ahead of Metallica’s closing two-hour headline set, which begins at 8.30pm.

Biffy Clyro headline Knebworth tomorrow (Saturday) with Slipknot closing the Apollo stage on Sunday night.