* BTCChina says to stop all trading on Sept. 30
* Said was due to recent regulatory announcements in China
* Rivals OkCoin, Huobi decline to comment (Adds market movement, quotes; paragaraphs 4-6, 9)
BEIJING/SHANGHAI, Sept 14 (Reuters) - Chinese bitcoin exchange BTCChina will stop all trading from Sept. 30, it said on Thursday, as regulators crack down on the nascent cryptocurrency sector in a bid to stamp out potential financial risks.
China has boomed as a cryptocurrency trading location in recent years, as investors and speculators flocked to domestic exchanges that formerly allowed users to conduct trades for free, boosting demand.
BTCChina, one of China's largest bitcoin trading platforms, which also runs an international exchange out of Hong Kong, will stop registration of new users from Thursday, it said on its official microblog.
"We will stop all trades on the digital trading platform starting Sept. 30," it added. Its co-founder, Bobby Lee, told Reuters the move would not affect trading on the BTCC international exchange, however.
The price of bitcoin and other cryptocurrencies tumbled after the news. By 1233 GMT, bitcoin was down 18 percent on BTCChina, at 20,510 yuan.
On U.S. exchange Bitstamp, it was down more than 6 percent at $3,632, down from its Sept. 2 record high of nearly $5,000.
BTCChina said its decision was based on a Sept. 4 directive from Chinese authorities that expressed concern over investment risks involved in cryptocurrencies and which ordered a ban on initial coin offerings.
Reuters and other media had reported this week, citing sources, that China planned to further ban exchanges that allowed virtual currency trading but the regulator has yet to make an announcement.
Spokeswomen for OkCoin and Huobi, BTCChina's main rivals in China, declined to say whether they would announce similar moves. Huobi said it had not received any clear directives from regulators to do so.
Investors in China contributed up to 2.6 billion yuan, or $397 million, worth of cryptocurrencies through initial coin offerings in January-June, state-run media have said, citing data from the National Committee of Experts on Internet Financial Security Technology.
The regulators' crackdown was sparked by fears that Chinese investors were speculatively piling into cryptocurrencies. Last week, they banned the practice of creating and selling digital currencies or tokens to investors to finance start-up projects.
($1=6.5520 Chinese yuan renminbi) (Reporting by Brenda Goh and Beijing Monitoring Desk; Editing by Nick Macfie and Clarence Fernandez)