South Korea's Samsung Tuesday announced the launch of its mobile payment service in China, where it is likely to struggle in an already crowded market that arch-rival Apple entered nearly six weeks ago.The world's second largest economy is also the world's biggest smartphone market, and Samsung Pay has a powerful partner in China - bank card provider UnionPay - which has links to the central bank. But unlike most other countries, mobile payment systems are already well-established in China, with local providers dominating the market. As an outsider Samsung will also have to compete with Apple, which launched its own Apple Pay system last month - also in partnership with Union Pay.The South Korean electronics giant said its service currently supports selected credit and debit cards from nine Chinese banks and is available on its top-end Galaxy smartphones, with mid-range models to follow. China had 359 million online payment users in mid-2015, up almost 18 percent in six months, according to the state-backed China Internet Network Information Center.