AMSTERDAM, Aug 17 (Reuters) - Shares in Dutch payments processor Adyen NV opened 19.8% lower on Thursday after first half earnings missed analyst estimates and the company's own ambitious targets.

Adyen, which provides online payment services for many of the world's largest internet platforms and retail stores, including Netflix, Meta, Microsoft and Spotify, citied slower growth in North America and ongoing hiring costs.

Under rules that suspend trading when a major price move is expected, the start of trading in Ayden shares was delayed by around 20 minutes on Euronext, which opens at 0700 GMT.

Earnings before interest, tax, depreciation and amortization (EBITDA) were 320 million euros ($348 million), down 10% from a year earlier and below analyst forecasts of 386 million euros, Refinitiv data shows.

Revenue rose 21% to 739 million euros, against Adyen's mid-term forecasts of more than 25% growth.

"In some areas the business grew at a lower rate than anticipated," the company's executives said in a letter to shareholders. "This was the case for our North American net revenue ... an increasingly important contributor in recent years."

The company also cited competition in the United States, where its rivals include Stripe, Fiserv and PayPal .

Adyen's EBITDA margin fell to 43% from 59%, which the company said was mostly because of higher wage costs as it takes on more staff.

A similar margin decline led to a sell-off in Adyen shares when the company reported full-year earnings in February.

"We are confident in the long-term benefits of building our team at an accelerated pace and consciously embrace its short-term impact," the company said on Thursday.

Adyen maintained its medium-term targets for revenue growth above 25% and an improving EBITDA margin that it expects to reach 65% in the long term. ($1 = 0.9205 euros) (Reporting by Toby Sterling Editing by David Goodman and Barbara Lewis)