By Alistair Barr

SAN FRANCISCO (Reuters) - Groupon Inc slumped on Friday as an initial public offering lock-up on stock sales by insiders of the world's largest daily deal company ended.

At one point the stock dropped more than 10 percent, triggering a circuit breaker that paused trading briefly and limited short sales.

Groupon sold a small amount of its equity in its IPO late last year. Insiders are typically prevented from selling for six months after an IPO.

Groupon's lock-up ended on June 1, making more than 600 million shares available to sell, representing more than 90 percent of its outstanding shares, according to Herman Leung, an analyst at Susquehanna Financial Group.

Expectations of such selling has pressured Groupon stock in recent weeks.

However, Groupon Chief Executive Andrew Mason said in May that he and the company's other founders were planning to keep their shares in the company after the lock-up expired.

Groupon shares were down 9.9 percent to $9.59 in afternoon trading on Friday. The stock has lost more than half its value since it debuted at $20 last year.

(Reporting By Alistair Barr; Editing by Tim Dobbyn)

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