(Reuters) - Wearable fitness device maker Fitbit Inc's revenue forecast for the key-holiday shopping quarter fell well short of analysts' estimates, hurt by stiff competition from rival device makers.

Shares of the company, which also reported lower-than-expected quarterly revenue, plummeted more than 30 percent to $8.93 in extended trading on Wednesday.

Fitbit forecast revenue of $725 million to $750 million for the key holiday-shopping quarter, well below analysts' estimates of $985.1 million, according to Thomson Reuters I/B/E/S.

That implies revenue growth of 5.4 percent at the top end of the forecast. Analysts were expecting growth of 38.4 percent, picking up from the latest third-quarter's 23.1 percent rise, which is the smallest increase since the company went public in June 2015.

"We continue to grow and are profitable, however not at the pace previously expected," Chief Executive James Park said.

Fitbit is the leader, according to research firm IDC, in the market for wearable devices such as wristbands that track the wearer's calories, sleeping patterns and heart rate.

But, the San Francisco-based company is facing increased competition from devices made by rivals such as Xiaomi, Garmin Ltd, Samsung Electronics and Apple Inc.

Fitbit's products have created excitement on Wall Street, but its stock has also attracted short sellers who are skeptical about the potential size of its emerging product category.

The short interest in the stock has steadily climbed to touch a record high of 26.6 percent of outstanding shares as of mid-October, according to Thomson Reuters data.

Fitbit, which launched two new fitness wristbands Charge 2 and Flex 2 in late August, said it sold 5.3 million devices in the quarter ended Sept. 30, up from 4.8 million a year earlier.

Analysts on average had expected shipments of 5 million devices, according to research firm FactSet StreetAccount.

Still, Fitbit's revenue of $503.8 million in the third quarter missed analysts estimates of $506.9 million.

Operating expenses jumped 52.4 percent to $196.2 million, largely due to an increase in research and development costs.

Net income fell to $26.1 million, or 11 cents per share, from $45.8 million, or 19 cents per share. Excluding items, it earned 19 cents per share, in line with analysts' expectations.

Fitbit's shares have fallen about 13.5 percent since Sept. 7 when Apple launched its latest smartwatch.

(Reporting by Laharee Chatterjee and Arunima Banerjee in Bengaluru; Editing by Savio D'Souza)