That day saw "the use of officers without identity badges, uniforms or vehicles marked as police cars actively shooting at protesters and arresting others", Irũngũ Houghton, Amnesty's Kenya director, told Reuters as the rights group published a press release detailing its reconstruction of the events.
The national police did not respond to Reuters' requests for comment on the Amnesty statement.
More than 50 people were killed in months-long protests over a controversial finance bill, creating President William Ruto's biggest crisis since taking office in 2022. Ruto abandoned the legislation in June and sacked almost his entire cabinet.
The rallies began peacefully, later turning violent. On June 25, the bill was scheduled for a third reading, and some demonstrators briefly stormed parliament. The police opened fire.
The rights group said its reconstruction of the day's events was conducted with five partners and based on interviews with 23 witnesses and analysis of dozens of videos and photographs.
As unarmed demonstrators entered parliament, men in civilian clothes were "seen on camera firing rifles and handguns towards the crowd and in the air", Amnesty said.
"Researchers counted at least 45 shots fired within 56 seconds."
Three witnesses saw at least six bodies of protesters who they believed were shot dead at the parliament complex, the rights group said.
"I saw my friend going down," said one person interviewed for the report, whose name was withheld. "He was shot."
Amnesty's report said images showed groups of men in civilian clothes carrying weapons and working alongside the police.
Amnesty also said Kenyan police that day unlawfully fired tear gas and beat and arbitrarily detained peaceful demonstrators, while some protesters threw back tear gas canisters at officers, hurled stones, broke windows and took flags.
(Writing by Paul Carsten; editing by Ammu Kannampilly and Alex Richardson)

Armed members of the Kenyan security forces fire teargas at demonstrators during a protest near the Parliament against tax hikes, in Nairobi, Kenya, 25 June 2024. Protesters have breached into the complex where parliament sits, setting fire to part of it, after lawmakers approved a finance bill that aims to raise an additional 2.7 billion US dollars in taxes as part of an effort to lighten the country's heavy debt load. At least five people died from gunshot wounds and more than 30 others were injured after protests turned violent, according to the Kenya Medical Association. EPA-EFE/STR