China has entered a "new era" where it should "take centre stage in the world", President Xi Jinping says.
The country's rapid progress under "socialism with Chinese characteristics" shows there is "a new choice for other countries", he told the Communist Party congress.
The closed-door summit determines who rules China and the country's direction for the next term.
Mr Xi has been consolidating power and is expected to remain as party chief.
The congress, which takes place once every five years, will finish on Tuesday. More than 2,000 delegates are attending the event, which is taking place under tight security.
Shortly after the congress ends, the party is expected to unveil the new members of China's top decision-making body, the Politburo Standing Committee, who will steer the country.
Listing China's recent achievements in his three-hour speech, Mr Xi said that "socialism with Chinese characteristics in this new era" meant China had now "become a great power in the world", and has played "an important role in the history of humankind".
The Chinese model of growth under Communist rule is "flourishing", he said, and has given "a new choice" to other developing countries.
"It is time for us to take centre stage in the world and to make a greater contribution to humankind," he added.
In his speech, Mr Xi also:
Described a two-stage plan for China's "socialist modernisation" by 2050, which would see it become more "prosperous and beautiful" through environmental and economic reforms
Warned against separatism - in an apparent reference to movements in Xinjiang, Tibet and Hong Kong - and reiterated the government's principle that Taiwan is part of China
Said China "would not close its doors to the world" and promised to lower barriers for foreign investors
He also introduced measures to increase party discipline, and touched on his wide-reaching corruption crackdown that has punished more than a million officials, report BBC correspondents in Beijing.
Beijing is decked out in welcome banners and festive displays for the congress.
However, the capital is also on high alert.Long lines were seen earlier this week at railway stations due to additional checks at transport hubs.
The congress has also affected businesses, with some restaurants, gyms, nightclubs and karaoke bars reportedly shutting down due to tightened security rules.
An austerity drive, instituted by Mr Xi, has meant a more pared down congress, with reports this week of delegates' hotels cutting back on frills such as decorations, free fruit in rooms and lavish meals.
Meanwhile, state media have said the Party is expected to rewrite its constitution to include Mr Xi's "work report" or political thoughts, which would elevate him to the status of previous Party giants Mao Zedong and Deng Xiaoping.
Some see Mr Xi as accruing more power than any leader since Mao, and the congress will be watched closely for clues on how much control now rests in the hands of just one man, says the BBC's John Sudworth.
Since becoming president in 2012, Mr Xi has tightened control within the Party and also in Chinese society, with increasing censorship and arrests of lawyers and activists.
Under Mr Xi, China's modernisation and reform has also accelerated, as has its assertiveness on the world stage.
He continues to enjoy widespread support among ordinary citizens in China.