China has tightened online policing this year, enacting new rules that require tech companies to store user data inside the country as well as restrictions on what is permissible content.

A WhatsApp user based in the southern China city of Guangzhou said she had had difficulty accessing social media and multiple VPN services since Sunday.

“I felt it might be a result of stepped up internet security measures to ensure a smooth party congress,” she said. “It’s possible that things will get worse as we draw closer [to the congress]. Things like WhatsApp, VPNs and overseas websites might be completely unavailable.”

Websites such as Facebook, Twitter, Pinterest and a slew of foreign media have been blocked for years on the mainland.

The WhatsApp troubles emerged ahead of the 19th National Congress of the Communist Party of China, which is set to start on October 18 and at which President Xi Jinping is expected to be given a second five-year term as the party’s general secretary.

“It smells like party congress pre-emptive blocking,” said Jason Ng, who researches China’s internet at the Citizen Lab at the University of Toronto.

China usually steps up surveillance around major events, he said.

While the WeChat messaging app owned by China’s Tencent is more widely used in the country, many WhatsApp users complained about the disruptions.

For many lawyers and activists, WhatsApp is the only viable option for communication because it is the only one that provides message encryption.

Rights lawyer Yu Pinjian said the latest disruption meant he had been unable to communicate with his friends and work contacts without fear of being monitored.

“Pictures and some group messages could not be received,” he said. “After WhatsApp was sabotaged, we were basically disconnected from the outside world,” he said.

Beijing-based activist Hu Jia shared Yu’s concerns.

“As we get closer to the party congress, I think the authorities will use more extreme censorship measures,” he said.

“Me and other dissidents use WhatsApp to communicate 70 per cent of the time. For the few days WhatsApp was completely inaccessible, we didn’t talk at all.”

The disruption to WhatsApp has also caused problems for the business community.

Allen Lai, a sales manager for a tile company in Guangzhou said the company relied heavily on the messaging program to communicate with overseas clients.

“I haven’t been able to message them on WhatsApp since yesterday, so we are using email for now,” he said.

“But if WhatsApp stays blocked in the coming days, I will have to ask them to switch to WeChat.”

Benny Mei, a salesman for a truck company in central China’s Hubei province, said his company was in a similar situation.

With clients in Africa and the Middle East, and the need to access Twitter and Facebook, he said the company had been forced to invest in a virtual private network, even though such services are banned in mainland China.

People on social media were quick to complain about the disruption to WhatsApp, saying it would damage business with foreign clients.

“Facebook, Twitter, Gmail, Viber were blocked before. Now even WhatsApp is blocked? Without good messaging tools, it will reduce the efficiency of foreign trade,” wrote a person on Weibo, China’s version of Twitter.

“I can live without the others [applications], but blocking WhatsApp is driving me crazy,” said another.

WhatsApp declined to comment.

Beijing has been steadily tightening its controls on social media and websites. Under the most recent rule changes, the operators of online groups will from next month be held responsible for the behaviour of their members and the information they publish.

Additional reporting by Viola Zhou