The Chinese authorities must immediately and unconditionally release all those detained for peacefully showing support for pro-democracy protests in Hong Kong, said Amnesty International.

At least 20 people have been detained by police in several cities in mainland China over the past two days for posting pictures online with messages of support for the protesters, shaving their heads in solidarity, or for planning to travel to Hong Kong to participate in the protests.

A further 60 have been called in for questioning by the authorities, known as being 'invited for tea'.

"The rounding up of activists in mainland China only underlines why so many people in Hong Kong fear the growing control Beijing has in their city's affairs," said William Nee, China researcher at Amnesty International.

"The fundamental freedoms being exercised by hundreds of thousands of people in Hong Kong continue to be denied to those in mainland China."

Police are known to have detained people in Beijing, Jiangsu, and the southern cities of Guangzhou and Shenzhen, which are close to Hong Kong.

"The Chinese authorities must immediately release all those being detained for peacefully expressing their support for protesters in Hong Kong," said William Nee.

Chinese state censors have attempted to block any mentions online of the pro-democracy protests. The popular photo-sharing platform Instagram was banned in mainland China on Tuesday.

Thousands of people continue to occupy parts of downtown Hong Kong to demand greater electoral reform.

The unnecessary use of tear gas and pepper spray by police at the weekend against an essentially peaceful protest only brought more people out on to the streets.

Police have in the last few days taken a less confrontational approach to the demonstrations since the weekend but there is much uncertainty about what will happen next.

"The Hong Kong police must show restraint in any response to the ongoing protest.

"Amnesty International urges them not to go down the route their counterparts in mainland China too often choose.

"They must respect the right to freedom of peaceful assembly and expression," said William Nee.