Using mobile scanners, police are taking copies of identity cards and anyone standing around outside the offices of the Southern Weekly newspaper is being checked.
After two days of protest it seems the apparent initial tolerance has come to an end.
The protest began when some reporting staff at the newspaper went on strike, complaining that an editorial in the New Year edition calling for greater rights in China was spiked and replaced with an article praising the achievements of the ruling Communist Party.
The switch had been made on the orders of the local propaganda chief here in the southern city of Guangzhou.
The angry response by the journalists who called the party official "dictatorial" in an open letter gathered support.
Celebrities with millions of followers sent messages backing the calls for freedom of speech and a free press.
This was rare. There are around 100,000 public protests every year in China but most are about land disputes and forced evictions.
This was a demonstration calling for political changes, in broad daylight, on the streets of a major city, which is unusual.
All the main search engines and web portals, the equivalent of Google and Yahoo in China were ordered to carry a pro government editorial, criticising the striking journalists.
They did post it but, in another rare act of defiance, added disclaimers saying that they did not share the views expressed in the article which blamed the protests on outside "hostile forces", including the blind activist Chen Guangcheng, who escaped house arrest last year and was allowed to move to the US.