* Xi opens Communist Party Congress, expected to cement control
* Twice-a-decade congress will select Politburo Standing Committee
* Xi opens congress with speech lauding anti-graft campaign
* Will Xi resurrect position of party chairman?
* Tight security across Beijing for congress
By Christian Shepherd and Stella Qiu
BEIJING, Oct 18 (Reuters) - Chinese President Xi Jinping opened a critical Communist Party Congress on Wednesday with a pledge to build a "modern socialist country" that will never copy the political systems of others and will remain open to the world.
Xi's wide-ranging speech kicked off the twice-a-decade congress, a week-long, mostly closed-door conclave that will culminate with the selection of a new Politburo Standing Committee that will rule China's 1.4 billion people for the next five years.
He set out his vision as he addressed more than 2,000 delegates in Beijing's cavernous Great Hall of the People, including 91-year-old former president Jiang Zemin.
"Through a long period of hard work, socialism with Chinese characteristics has entered a new era, this is a new historical direction in our country's development," Xi said in a speech carried live across the nation on state television.
China will relax market access for foreign investment and expand access to its services sector, as well as deepen market-oriented reform of its exchange rate and financial system, while at the same time strengthening state firms, he said.
China's political system is the broadest, most genuine, and most effective way to safeguard the fundamental interests of the people, Xi said.
"We should not just mechanically copy the political systems of other countries," he said.
Xi praised the party's successes, particularly his high-profile anti-graft campaign.
"The fight against corruption has formed an overwhelming posture and strengthened in development," he said.
Xi has waged a relentless fight against deep-rooted graft since assuming power five years ago, with more than a million officials punished and dozens of former senior officials jailed.
He also said China had firmly opposed and prevented independence for self-ruled Taiwan, claimed by Beijing as its own, over the past five years.
While the speech was high on aspiration and short on policy specifics, its language will be parsed for signals or policy directives, including on the direction of reforms for the world's second-largest economy.
Xi has consolidated power swiftly since assuming the party leadership in 2012, locking up political rivals for corruption, restructuring the military and asserting China's rising might on the world stage.
Focus at the congress will be on how Xi plans to put his expanded authority to use.
Key questions include whether Xi ally and top corruption-buster Wang Qishan will stay on past traditional retirement age and to what extent Xi will promote allies to senior positions.
Close attention will also be paid to any moves that would enable Xi to stay on in a leadership capacity after his second term ends in 2022. That could include resurrecting the position of party chairman, a title that would put him on par with Mao Zedong.
One of the most important signals for that would be whether - or how often - Xi is referred to as "lingxiu", or leader. That honorific has been bestowed on only two others since the founding of the People's Republic of China in 1949: Mao and his short-lived successor, Hua Guofeng.
Such decisions will be formally announced at the end of the congress next week.
Another key measure of Xi's power will be whether he manages to have his name "crowned" in the party constitution, elevating him to the level of previous leaders exemplified by Mao Zedong Thought and Deng Xiaoping Theory.
As with other major set-piece events held in the capital, Beijing has been blanketed with security, with long lines at some suburban subways stations as passengers waited to go through metal detectors and be patted down.
Coverage in state media and across the city has kicked into overdrive, with large red banners plastered around Beijing welcoming the congress, while censors have stepped up already tight monitoring of the internet.
Tencent Holdings Ltd's WeChat, China's top social media platform with more than 960 million users, released a short statement late on Tuesday saying that, due to "system maintenance", users will be unable to modify profile pictures, nicknames and tag lines until the end of the month.
The disabled features are sporadically used to show solidarity for popular social and political causes.
Tencent recently ramped up controls on chat services, making users legally liable for content deemed offensive to Communist Party values, as well as updating the firm's privacy policies to clarify that message data is made available to government agencies.
Additional reporting by Philip Wen, Michael Martina, Cate Cadell, Kevin Yao and Elias Glenn; Writing by Ben Blanchard; Editing by Paul Tait