An outspoken former property tycoon disciplined by China's ruling Communist Party for questioning its media controls made a rare public appearance on Sunday, months after authorities shut down his hugely popular microblog.

The government-ordered closure in February of Ren Zhiqiang's Sina Weibo account, which had some 37 million followers, was seen as a sign of ever-tighter restrictions on freedom of speech under President Xi Jinping.

Ren, nicknamed "The Cannon" for his outspoken opinions, did not mention the incident in a speech at a forum on environmentalism delivered to dozens of media and entrepreneurs.

"We should say no to firms which pollute the environment during the production process," Ren said in a speech lasting some 20 minutes and broadcast online.

Asked by an AFP reporter if he could discuss his microblog's closure, he replied gruffly: "I can't."

The Cyberspace Administration of China ordered the closure of Ren's blog after he questioned Xi's policy of tighter party control over state media.

"When did the people's government change into the party's government?" he wrote "Is their money the party's? ... Don't use taxpayers' money for things that don't provide them with services."

The party, of which Ren is a member, later announced he would be suspended for a year over the posts.

Ren stepped down as president of property firm Beijing Huayuan Group in 2011. One report estimated his net worth in 2009 at around $22 million.

Ren last week gave his first online interview since the microblog's closure, in which he called on the government to deepen market reforms and obliquely referred to his own punishment.

"There are some activities the upper ranks won't let me take part in," he said. "But when I take part in philanthropic activities, they will allow it."