Pictures posted online showing visitors throwing snowballs at lions have triggered heated debate on the welfare of zoo animals.

The pictures, taken at the lions' enclosure in Hangzhou Zoo on Jan 5, were posted on Sina Weibo, the Chinese Twitter-like service, the same day.

They were reposted almost 80,000 times, with thousands of comments left by angry Internet users criticizing the visitors' behavior.

One user, named "Sina Zhejiang", said in the post that as some visitors kept throwing snowballs at two lions, they had nowhere to hide and curled up in a corner, shaking.

A manager at the zoo was quoted by Xinhua News Agency on Monday admitting there had been negligence by the management.

But the manager said the zoo had no right to fine visitors who abused the animals. What they could do is to call for better treatment of these animals.

Mang Ping, professor with the Central Institute of Socialism, who has studied the welfare of zoo animals since 2003, said abuse of zoo animals by visitors or zookeepers was commonplace in China. But the law cannot protect zoo animals, because the definition on wild animals is not clear.

Mang and 50 other legal experts wrote to the State Council, China's cabinet, on Tuesday, calling for stricter law enforcement and further regulation of wild animals’ welfare.