Gwengi told the Committee to Protect Journalists that police detained him as returned to his home in Usenge, on Lake Victoria.
"They picked me up on the street as I was leaving for home. They handcuffed me before they beat me. Then I was taken to the police station where I spent the night. I was released the following morning," he said.
Seven officers from the national Administrative Police force beat him, leading to his brief hospitalization, Gwengi told CPJ.
"They told me they will ensure I am dead, [that] if I want I cannot fight them using a pen or a notebook, I can only fight them by acquiring a gun," he said.
Gwengi said he believed the attack was retribution for his articles criticizing alleged police misconduct. In February 2016, for example, he reported on allegations of police extortion and sexual assault.
"I've been doing stories on human rights violations by these Administrative Police officers. These stories have been exposing [them]. So they've not been happy with these stories," Gwengi told CPJ.
He said he began receiving verbal threats from police through intermediaries after the articles were published. The threats escalated and a police officer directly threatened him in January 2017, the journalist said.
"One officer personally told me that I either stop writing about them or I will face unspecified consequences," Gwengi said. "That is the reason I was taking precautions. I knew these people were hunting me. They tried all means getting me. I wasn't hiding but I was taking some safety measures. When I go home, I don't go home direct. Sometimes [I took a] motorbike. Sometimes I walked, sometimes I don't stay in house."
Gwengi told CPJ that he had filed a complaint with local police in Usenge. Administrative Police officers denied the journalist's allegations and filed incitement charges against him, the journalist told CPJ.
The Standard reported that police complained that Gwengi had written articles portraying them as "as inept and corrupt, and that this had resulted in loss of public confidence."
Nyanza Regional Police Commander Assistant Inspector General Willy Lugusa told CPJ that police were investigating Gwengi's complaint as well as the charges made against the journalist.
Gwengi told CPJ that a court in Usenge was due to hear his case on March 28 and April 6, but postponed both hearings pending further investigation.
The Media Council of Kenya, a government-funded regulator, called for the officers to be arrested, The Star newspaper reported on March 23.
"This is unacceptable," The Star quoted Media Council chair Charles Kerich as saying. "As we head to the election, brutality will not be condoned."