Kenyan human rights activist Wanjeri Nderu has tweeted footage of county security guards beating up a woman who is thought to be trader.
A source has confirmed to the BBC that the assault happened in the town of Kitengela in Kajiado County, about 30km (18 miles) south of the capital, Nairobi.
The person who recorded it says when the security guards noticed her filming them using her mobile, they turned on her – beat her up and confiscated one her phones.
The Kajiado county government says it is investigating the matter.
The area where the assault happened is in a market area in the town that the county authorities are trying to refurbish.
Traders have been complaining that the project is taking too long, and some have been coming back to set up stalls by the roadside.