According to the Hong Kong Free Press news site, at least 4,000 people gathered on Sunday to protest, although the Police had warned them not to do so. Police officers threw pepper spray at demonstrators. Besides the four arrested, two policemen were wounded.
The controversy began on October 12 after pro-sovereignty Youngspiration members Baggio Leung and Yau Wai-ching, who had been elected in the September 4 election, read their oaths of office.
In doing so, Leung and Yau used an unofficial formula which included derogatory terms towards China, and unfurled a banner which read: "Hong Kong is not China."
Andrew Leung, Chairman of the LegCo, did not accept their oaths as valid, and brought the case to a Hong Kong court.
But Beijing decided to intervene in the dispute. The last week it came to be known that the Standing Committee of the National People's Congress —China top legislative body— would be taking a final decision on the issue.
This led thousands to launch street protests over the weekend. Protesters oppose to the fact that Beijing intervenes to interpret whether Hong Kong laws admit or not that an elected lawmaker can use an alternative oath formula, different to that established by law.
What protesters essentially fear is Beijing's increasing grip on Hong Kong politics. The former colony maintains a semi-autonomous status within China, but as years go by the Chinese government is showing that it wants to limit Hong Kong self-government.
Most Hong Kong voters support parties in favour of maintaining or even increasing the city's autonomy. But the particular electoral system of the former colony gives pro-Chinese parties a majority of the LegCo seats.
More police officers to the streets after Beijing's decision
In anticipation of further protests in the days to come, the Hong Kong Police has prepared the deployment of 2,000 agents, the South China Morning Post reports quoting police sources.
Protests could be unleashed again after the Standing Committee today approved a legal interpretation according to which it is understood that an elected lawmakers "declines" to take up his or her office if he or she does not used the legally established oath formula, or if the oath is not performed "sincerely and solemnly."
This interpretation de facto implies that Yau and Leung will be effectively barred from office.