Uniformed officers are alleged to have held demonstration signs and given interviews at the protest, expressing their support for the movement.
Finland is investigating complaints that Helsinki police took part in a Black Lives Matter demonstration last month.
Uniformed officers allegedly held demonstration signs and gave on-site interviews in support of a protest in Senate Square on June 3.
Complaints criticised the police for their apparent impartiality and for allowing thousands of citizens to demonstrate, despite COVID-19 pandemic restrictions.
On May 19, the regional government in Finland banned public gatherings of more than 500 people until June 30, due to safety and hygiene concerns.
Parliamentary Ombudsman Petri Jääskeläinen said in a statement that 16 total complaints had been filed against the Helsinki Police Department.
Three other anonymous reports of improper police conduct have also been made via an internal ethics channel.
Police presence to "monitor public order and security"
The police killing of George Floyd in the United States on May 25 triggered numerous protests around the world against police violence and social injustice.
Around 3,000 people gathered peacefully in Helsinki on June 3, chanting slogans such as "Black lives matter" and displaying "I can't breathe" signs.
A preliminary report has indicated that Helsinki police were informed two days previously that a demonstration would take place in Senate Square.
A tweet from Helsinki Police on June 3 confirms that officers were in attendance to "monitor public order and security".
But police said that they were informed that the number of participants at the protest would be "within the allowable limit".
The authorities requested organisers to stop the demonstration "as soon as possible" due to government restrictions and the demonstration was called to end after around one hour.
After this protestors continued to move through the city under police control until a "small number" gathered at the steps of Parliament.
Complainants allege a police double-standard
Some complaints about police conduct alleged that officers had not been balanced in their enforcement of government restrictions.
On May 8, Helsinki police had intervened when 12 people gathered in front of Parliament House, protesting "for the establishment of the Constitutional Court".
Authorities called for a reduction in the number of participants, as gatherings were, at the time, limited to 10 people under the Regional State Administrative Agency.
But Helsinki Police said in a statement that the demonstration on May 8 was "essentially illegal" because the authorities had not been notified in advance.
Police also say that the organiser of this protest did not agree to make the gathering lawful and was fined for violating assembly rules.
Two other people were also arrested at the demonstration, according to Chief Commissioner Seppo Kujala.
Finland's Parliamentary Ombudsman has asked for more information on how police had prepared for the Black Lives Matter protest on June 3 and why this demonstration was allowed to continue.
In a statement to Euronews, Helsinki Police Department said they support the investigation and wait for the ombudsman's decision.
Finland's Interior Ministry has also confirmed that investigations by the Parliamentary Ombudsman concern complaints where it is suspected that "an authority or official has not observed the law or fulfilled a duty, or if the complainant suspects that fundamental and human rights have not been appropriately implemented."
No official statement on the investigation has been released due to ongoing inquiry. Finland's Interior Ministry has until October 31 to file an opinion.