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You had to get up pretty early to get a free Shake Shack burger

Shake Shack's free burgers sell out early

If you were hoping to score a free burger from Shake Shack, even late breakfast time was a little too late.

The burger chain celebrated the opening of its 100th restaurant on Tuesday by giving away free burgers to the first 100 customers at each of its locations. By lunch time, some were running out of burgers. Most restaurants charge around $6 a burger.

Customers across the country lined up outside of Shake Shack chains hoping to nab a voucher for a free burger. Many of the lines spilled out onto the street.

While many customers took to Twitter to celebrate the company's promotion, others complained that burgers were given out before the 10:30 a.m. start time that Shake Shack had advertised.

"We gave out burgers a bit before 10:30 a.m. at Shacks that serve breakfast (like Fulton Center and Grand Central) where we saw guests lining up at the wee hours of the morning," a Shake Shack spokesman told CNBC.

For those who did not receive a free burger, Shake Shack also gave out free fries and custard.