Hundreds of workers marched past the factory on Wednesday and faced police, who were armed with batons and dressed in full white protective suits.
In the aftermath, Zhengzhou authorities ordered a major screening campaign in several districts of the city. These neighborhoods are confined for five days from Friday midnight.
Residents of the city center are no longer allowed to leave the area without a negative PCR test and permission from local authorities. They are advised not to leave their homes, "unless necessary".
These measures concern several districts, affecting about half of the inhabitants of Zhengzhou. They do not cover the area of the iPhone factory, where workers have already been subject to restrictions for several weeks.
A worker told AFP that the protests began due to a controversy over the amount of a bonus promised to workers at the factory, owned by Taiwanese giant Foxconn, Apple's main subcontractor.
According to this worker, the bonus would have gone from 3,000 to 30 yuan (from 400 to 4 euros), which would have caused great dissatisfaction among the staff.
Several employees are also exasperated by the "chaotic" living conditions on the site due to anti-Covid restrictions, said this worker, who wishes to remain anonymous to avoid possible reprisals.
- "Technical error" -
Foxconn apologized on Thursday, saying a "technical error" in its payment system was the cause of the problem around the bonuses. The group said it "fully understands" the workers' concerns.
"The company will also do its best to proactively resolve reasonable employee concerns and requests," the Taiwanese company said.
The American giant Apple, whose iPhone is the flagship product, told AFP on Thursday that it has representatives currently present on the site.
The California group is "reviewing the situation and working closely with Foxconn to ensure their employees' concerns are addressed," he said.
Last month, workers, panicked by the unexpected confinement of the factory, had fled the site on foot.
Under China's strict "zero Covid" health policy, the slightest increase in cases leads to the closure of entire cities and the placing of those who test positive in quarantine.
But this strategy is now causing the weariness of a large part of the population. And the number of infections is skyrocketing.
The Ministry of Health announced Thursday a new record for the number of people tested positive in 24 hours, at 31,444 local cases - including 27,517 asymptomatic.
This number exceeds the previous record (29,317) recorded in mid-April, when the metropolis of Shanghai was confined.
- Makeshift hospitals -
However, these figures appear low in view of the Chinese population (1.4 billion inhabitants) and the records observed in Western countries.
Several other major Chinese cities, including Beijing, Shanghai and Chongqing, have also tightened restrictions.
The capital now requires a negative PCR test of less than 48 hours to be able to enter public places such as shopping malls, hotels and administrative buildings.
Many schools have switched to distance learning. Most restaurants, bars and shops are closed.
The manufacturing center of Canton (south), epicenter of the current wave of Covid, has built thousands of temporary hospital rooms to accommodate patients.
The government announced on November 11 an “optimization” of anti-Covid restrictions, with the relaxation of several measures, including the reduction of the duration of quarantines, in particular for travelers arriving from abroad.
But Shijiazhuang, a township neighboring Beijing that was seen as a pilot city to test this reopening strategy, has backtracked on most of the relaxations - as have other localities.
"The road to reopening could be slow, costly and bumpy," said Ting Lu, economist for Nomura Bank, in a note.