HONG KONG, Dec 6 (Reuters) - Shares of China Evergrande Group (3333.HK) tumbled 12% to an 11-year low on Monday after the firm said there was no guarantee it would have enough funds to meet debt repayments, prompting Chinese authorities to summon its chairman. The shares fell as a 30-day grace period on a coupon payment of $82.5 million due on Nov. 6 comes to an end on Monday.Evergrande, once China"s top-selling developer, is grappling with more than $300 billion in liabilities. A collapse could send shockwaves through the country"s property sector and beyond. In a filing late on Friday, Evergrande, the world"s most indebted developer, also said it had received a demand from creditors to pay about $260 million.That prompted the government of Guangdong province, where the company is based, to summon Evergrande Chairman Hui Ka Yan, and it later said in a statement it would send a working group to the developer at Evergrande"s request to oversee risk management, strengthen internal controls and maintain normal operations. In a series of apparently coordinated statements late in the evening, China"s central bank, banking and insurance regulator and its securities regulator sought to reassure the market that any risks to the broader property sector could be contained. Short-term risks caused by a single real estate firm will not undermine market fundraising in the medium and long term, the People"s Bank of China said, adding that housing sales, land purchases and financing "have already returned to normal in China". Evergraned"s stock fell more than 12% to HK$1.98, its lowest since May 2010.