BEIJING, China -- Millions of people in areas surrounding China's capital were ordered into lockdown Tuesday, with authorities doubling down on efforts to contain Covid-19 ahead of a key ruling Communist Party meeting this year.

Nearly four million people in Hebei province, which surrounds Beijing, were ordered to stay home until the end of the week as officials rush to curb a small virus flare-up.

And more than 13 million in the neighbouring port city of Tianjin must undergo mass testing from 6 am (2200 GMT), after 51 mostly mild cases were reported.

China is the only major global economy sticking to a zero-Covid policy, and lockdowns, travel restrictions and mass testing have disrupted businesses and cooled growth.

Beijing has doubled down on the policy ahead of the Communist Party's 20th National Congress, expected to take place within the next three months.

The handling of the pandemic is widely seen as central to the political legacy of President Xi Jinping, who is set to be anointed for an unprecedented third term in office at the meeting.

But "the actual Covid situation in China might be worsening, as Omicron has once again spread to large cities", Nomura analyst Ting Lu warned in a research note.

Chengdu, western China's largest city, has also seen strict travel restrictions.

And in the southern tech hub of Shenzhen, two districts are under partial lockdown, while the world's biggest electronics market in Huaqiangbei has closed -- despite just 35 daily cases being reported in the city of over 18 million.

"The notice to close came abruptly, we only had a few hours to put our stock into warehouses and lock up," a trader at the Huaqiangbei tech market, who only offered his last name Chen, told AFP.

In Futian, where the city government is located, cinemas, karaoke bars and parks are closed until Friday and large public events have been cancelled.

Officials also sealed off Wanxia neighbourhood on the city's outskirts, which offers affordable housing for delivery drivers and migrant workers, although no cases were reported there.