More than 200 Chinese citizens stepped up to a mock ballot box on Wednesday, overwhelmingly "re-electing" US President Barack Obama while reflecting ruefully on the lack of multi-party democracy in China.

The world's two biggest economies are choosing their leaders around the same time, a quirk of timing that highlights the stark contrast between China's secretive communist system and America's boisterous democracy.

The "voters" were among several hundred Chinese invited to join American citizens in the balloting at an election party organised by the US embassy. Obama emerged the landslide winner with 153 votes to 51.

"It is unfortunate that China cannot have elections like this," a beaming He Jiangtao told AFP after casting his vote.

"As long as the Communist Party is in power, it is unlikely that China will ever have democratic elections," said He, a marketing consultant.

While Obama was re-elected to another term before the eyes of the world, China will choose its next leaders behind closed doors at its five-yearly party congress -- the 18th in the country's history -- opening in Beijing on Thursday.

At the week-long congress, President Hu Jintao and Premier Wen Jiabao will step down from their top party posts to make way for a new leadership that will -- early next year -- take the reins of the world's most populous nation for the next decade.

Chinese citizens have no say in that choice and the mock vote thrilled some participants.

"I voted for Obama. I support the Democratic Party because it is very liberal. For me, they better represent American freedom," Wang Zheng, manager of a travel agency, told AFP.

"You can see how excited and happy the supporters of Obama are," Wang said, pointing to celebrations in the United States broadcast on large-screen televisions.

"You won't see anything like this after the 18th party congress, people will not be expressing their happiness like that," he said.

In parts of China, villagers can directly elect their village chiefs from tightly controlled lists of candidates. But state, provincial and national leaders are all appointed by the ruling party.

With little experience in elections, the Chinese listened as embassy staff carefully explained the US electoral system involving simultaneous voting for Congress and various ballot initiatives.

Voters also lined up to take pictures with life-size photos of Obama and Republican candidate Mitt Romney as television screens beamed live election coverage from major US networks.

"The world is looking to see if China will have something new to offer with the economy and with its politics, so the (Communist Party congress) should show what road China will go down," said He Jie, an official at a human resources agency.

He echoed growing calls by some Chinese intellectuals for political reform.

"I hope they (the Communist Party) can better listen to the opinions of the people, and react to what the people want and do it for them, instead of always making decisions by themselves on what should be done," he said.

China's outspoken netizens expressed similar sentiments.

"Congratulations to Mr Obama for being successfully re-elected as US president," said one user of a popular micro-blogging service.

"Someone asked: What is the difference between elections in America and China? The former has genuine democracy, the latter is fooling the people. Long live the 18th party congress!"

China, which has the world's largest number of Internet users at more than 500 million, censors Internet content it deems politically sensitive. But authorities have found it challenging to keep up with postings in Twitter-like microblogs.

Officially, China congratulated Obama on his re-election, noting "positive progress" in relations in his first time.

"President Hu Jintao and Premier Wen Jiabao sent messages of congratulations to President Obama on his being re-elected president of the United States," said foreign ministry spokesman Hong Lei.

China will "look to the future and make continuous efforts for fresh and greater progress in the building of the China-US cooperative partnership".