Organisers had planned marches to mark the second anniversary of the unrest, when protesters breached parliament grounds before a crackdown left at least 60 people dead, according to the Kenya National Commission on Human Rights.
A Reuters correspondent saw police fire tear gas to scatter people gathering peacefully at Nairobi's main police station on Thursday after security forces detained at least six outside parliament, where they had laid flowers.
Footage broadcast on Citizen TV also showed police firing tear gas on the outskirts of Nairobi.
In Mombasa, dozens of marchers had earlier been escorted by security forces.
But in the capital, shops and restaurants within the central business district had remained shut as police set up roadblocks with water cannon trucks, and cordoned parliament off with razor wire.
Opposition leaders joined victims of alleged police brutality and families of slain protesters heading to parliament.
"As parents, we sought permission just to come here ... to mourn and lay flowers for our children. But when we arrived, we were shocked because the police blocked us," said Edith Wanjiku, whose 19-year-old son Ibrahim Kamau was killed in 2024.
"That is very shameful."
"And one thing I would ask of President (William) Ruto: those police officers who killed the children - because they are known - I am only asking for justice for those children and also compensation."
Interior Minister Kipchumba Murkomen had said anyone seeking to engage in "chaos, looting, destroy property, disrupt businesses, or commit any other criminal acts" would be dealt with severely.
Protest organisers say they want credible investigations into past police conduct and guarantees against the use of excessive force.
Ruto has acknowledged "instances of excessive and extrajudicial actions by members of the security services", and said two billion shillings ($A22 million) had been set aside for victims of protest-related abuses.
However, civil society organisations want more.
"Every time there is a protest, innocent Kenyans are killed. Because these killer cops are not being arrested. They are not being held to account," activist Hussein Khalid said.
"We will continue pushing for justice ... they will not hoodwink us with money."
