Article 14 of the Basic Law stipulates that mainland Chinese military forces "shall not interfere in the local affairs of the Region", but that the Hong Kong government may, when necessary, ask for assistance "in the maintenance of public order and in disaster relief". Masked protesters with makeshift weapons barricaded themselves inside the Chinese University of Hong Kong on Wednesday after violent clashes with riot police overnight.
Protesters sprayed with the blue liquid stripped down to their underwear and hosed one another down to wash it off, not knowing what it contained.
But the message seemed to have little effect - and the protesters remain inside the campus.
"The protesters have been reacting to the police", said Joris, 23, a civil engineer who like others did not give his full name.
The choreographed troop movement "has only compounded the impression that Beijing has simply ignored" Hong Kong's unique political system, said analyst Dixon Sing.
On Sunday, a member of police staff was hit in the leg with an arrow apparently fired by a protester from a bow.
Hong Kong saw a peaceful rally and a violent clash on Sunday, between young protesters and police.
Dozens of protesters tried to leave after sunrise but turned back as police fired tear gas and rubber bullets.
Three protesters have been shot by armed police in the unrelenting months of protests.
"We want to do our part and come back to help clean up while it's not so tense today". The fire brigade moved in after about half an hour to douse the flames.
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A low-frequency "woo-woo" sound could be heard coming from a police armoured vehicle on Sunday, according to reporters covering intense clashes between radical anti-government demonstrators and police near Polytechnic University in Hung Hom.
Fiery explosions could be seen inside as riot officers entered before dawn on Monday.
Chinese soldiers from the Hong Kong garrison are rarely seen outside - especially on the street in formation - and any deployment is likely to heat up tensions in the city.
Although the proposed measure has now been withdrawn, public anger has been driven by the authorities' refusal to address protesters' demands for an independent inquiry against alleged police brutality; amnesty for the almost 600 people charged with offences stemming from the protests; a retraction of police claims that protesters are guilty of rioting; and universal suffrage to elect the full legislature and chief executive.
"We are here for the long term", a 20-year-old student who identified himself only as E said. "If we don't persist, we will fail".
The violence, which has convulsed the Asian financial hub, has posed the gravest popular challenge to Chinese President Xi Jinping since he came to power in 2012.
The presence of soldiers from China's People's Liberation Army (PLA) on the streets, even to clean up, risks stoking the controversy about Hong Kong's status as an autonomous area.
Protesters are angry at perceived Communist Party meddling in the territory, whose freedoms were guaranteed when it returned to Chinese rule in 1997.
Opposition lawmakers criticized the Chinese military for joining a cleanup to remove debris from streets near Hong Kong Baptist University on Saturday.
District council elections scheduled for November 24 are seen as a barometer of public sentiment in Hong Kong, which has been riven by anti-government protests for more than five months.