(CNN) -- Prosecutors are investigating police actions during a protest earlier this month in which at least 16 people died, the Office of the General Prosecutor in Kazakhstan said Thursday.
At least 80 people were also injured in the December 16 clashes between police and striking oil workers in the oil town of Zhanaozen, according to state media.
A 20-day state of emergency was declared following the unrest, which also spilled over to a nearby village, where at least one person died as protesters blocked a passenger train in a show of support for the oil workers.
A spokeswoman for the Organization for Security and Co-operation in Europe, Virginie Coulloudon, said Kazakhstan had told fellow OSCE states Tuesday that the general prosecutor's office had started its own inquiry into police actions.
Kazakh authorities said the internal security department of the Ministry of Internal Affairs had also launched an in-house investigation into claims of firearms use by the police, Coulloudon said.
"The conclusions of this investigation, according to Kazakh sources, should be available within a week," she told CNN via email.
The Kazakh general prosecutor's office said in a statement on its website Thursday that 20 people had been detained over involvement in the unrest in Zhanaozen, with 18 of the arrests ordered by the court.
A criminal case was opened Tuesday into the alleged excessive use of force by police who opened fire, it said, but some of the deaths reported from Zhanaozen were not related to the actions of law enforcement.
The investigation will be headed by a special prosecutor in order to guarantee its objectivity and measures have been adopted to ensure transparency, the statement said.
A separate committee has also been formed, with relatives of those detained among its members, to examine claims of unlawful detention and abuse of detainees, it said.
At least one person is being investigated for giving "false information" to the media about events in Zhanaozen, the prosecutor's office added.
The trouble came as the former Soviet republic celebrated its 20th anniversary of independence and prompted concern that unrest might spread across the oil rich Central Asian nation.
The protest was part of a long-running dispute over low pay and the sacking of some workers.
Video posted to YouTube appeared to show protesters in Zhanaozen fleeing a large square amid gunfire, as police advance with riot shields. An injured person is seen being beaten with a baton by what appears to be a policeman.
The workers' trade union puts the number of dead at 50 to 70 and says as many as 500 were injured, according to European lawmakers.
International rights groups have condemned the violence and subsequent emergency measures imposed by the government, including curbs on communications and freedom of movement, and called for a full investigation.
The U.S. State Department and a group of 48 European lawmakers also expressed concern.
The Kazakh authorities announced Wednesday that 1 million Kazakh tenge ($6,640) would be paid to the families of each of those killed and half that sum to those injured.
Analysts say the Kazakh government is keen to prevent discontent over increasing social inequality, in a country where most ordinary people have not shared in the wealth brought by oil.
Parliamentary and local elections are due to take place next month.
Kazakhstan has often boasted of its stability in a region that has seen its share of conflict. The ninth-largest country in the world by area, it has the largest economy of all the Central Asian states thanks mostly to its natural resources, according to the CIA World Factbook.
CNN's Alla Eshchenko, Sarah Jones and Laura Smith-Spark contributed to this report.