Four members of the pro-democracy groups that organize Hong Kong's annual Tiananmen Square vigil were arrested by Police Wednesday. The members were arrested after they failed to comply with a police order for them to disclose information about their organization.

The order issued last month demanded that the Hong Kong Alliance in Support of Patriotic Democratic Movements of China submit to authorities information such as funding sources and membership names.

Authorities said the demand was made on national security grounds and over reports that the group was working as a "foreign agent" for another country, which is in violation of the recently imposed national security law.

According to the demand letter, the group had only 14 days to submit the information. Failure to comply would result in members being imprisoned for up to six months.

The group said among those that were arrested were its vice chairwoman, Chow Hang-tung, and standing committee members Chan Dor-wai, Leung Kam-wai, and Tang Ngok-kwan. Prior to the arrest, Chow was able to post on social media about police officers arriving at their office.

Police confirmed that they had arrested the four individuals for violating the city's national security law. Officials added that more people could be arrested as they are still planning to conduct further operations.

The group, which was formed more than three decades ago, remains to be one of Hong Kong's largest pro-democracy groups. The alliance said it had refused to comply with the order as it included the disclosure of private information such as personal details of its members and records of its meeting with other groups overseas, including with those in Taiwan.

A day prior to the arrest, the alliance had sent a letter to police arguing that they had no grounds to request private information. In the letter, the group also denied allegations that it was acting as a "foreign agent" for another government. The group also challenged the police to show evidence linking them to foreign powers.

Hong Kong's security agency had sent several warnings to the group, asking them to "turn back before it is too late." The agency accused the group of being a danger to the country's national security, which it said was a "very serious crime."