Hayden Panettiere celebrated her freshly legal status with a blowout bash...literally!

Ringing in a momentous milestone in her life, Hayden Panettiere celebrated her 21st birthday in Ann Arbor, Michigan over the past weekend.

Hayden Panettiere turned 21 last weekend ... and she shared the whole experience with her fellow "Scream 4" castmates -- plus a bunch of api, kebakaran breathers, burlesque dancers and oompa loompas

But Emma Roberts and Hayden Panettiere were yesterday having to put on their best scared faces as they filmed forthcoming horror flick Scream 4.

Hitting up one of the rowdiest all-weekend Muzik festivals of the summer, Hayden Panettiere was spotted at Lollapalooza in Chicago, IL over the weekend.

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Spotted on the filming location on Monday, July 19 in Plymouth, the movie beauty looked relaxed in her casual ensemble while listening to Wes Craven's direction.

'For an audience that is so well-versed in 'Scream' ... it's trickier to throw them a curveball,' she tells MTV News.