Chinese authorities say police have arrested 15,000 people on suspicion of cybercrime, including hacking, online fraud and illegal sale of personal information.

The Ministry of Public Security said on Tuesday that police have investigated more than 7,400 cases of alleged online crimes, leading to the arrests. It did not say when the arrests were made.

Authorities launched a six-month special operation to "clean the internet" in July, amid a growing crackdown on public dissent.

The ministry said the special operation has snared suspects who hacked into websites of companies, banks and government agencies.

Some obtained personal information illegally, some altered web information or uploaded content related to online gambling, and some used the internet to defraud others, it said.

China also prosecutes improper online speech, although the ministry's announcement did not list cases involving that.

Possible causes

Xiao Qiang, a professor at the UC Berkeley School of Information in California, told Al Jazeera he had never seen a crackdown on cybercrime at this level in China.

It was possible that some of those arrested could be activists, with cases related to freedom of expression, he said.

"The general atmosphere is very tight when it comes to freedom of speech," Xiao said.

He said the announcement of the mass arrests, which come amid a public outcry over the August 12 explosions at a warehouse that killed more than 100 people in Tianjin, could also be as an indirect warning against dissent.

The reported arrests come just three days after the Cyberspace Administration of China - the country's main internet watchdog - said that it shut down 18 news websites indefinitely and temporarily suspended 32 other sites for "spreading false rumours" about the Tianjin blasts and "inciting panic".

China has been accused of responding harshly to allegations that government mismanagement led to the disaster in the port city.

The Associated Press news agency says at least one person has been detained for allegedly reporting an exaggerated death toll of the blasts.

Crackdown on lawyers

Earlier, more than 200 Chinese lawyers, bloggers and activists were targeted in what Amnesty International, the international human-rights monitor, called an "unprecedented" spree of detentions and police questioning.

The public security ministry was cited as calling Beijing's Fengrui Law Firm - a prominent practice with leading human rights lawyers - a "major criminal gang" that had engaged in "disturbing social order."

The People's Daily reported that the crackdown began when the public security ministry looked into the actions of two activists, later broadening their investigation.

"The Chinese authorities must end their assault on human rights lawyers," Amnesty International said last month after more than 50 lawyers and activists were targeted by police since July 9.

Since the crackdown, a group of prominent lawyers in Hong Kong has launched a global petition to denounce what it calls the "intimidating harassment" of lawyers by authorities.

With reporting by Ryan Rifai