Students and teachers returned to a Nairobi-area school from a break to find that a barrier had been erected around their play area by a private developer in an apparent sale of the land.

The Nairobi city council has not commented on the legality of the sale of the land it says is public, though some protesters allege it was an unlawful land grab. Around 100 students, mostly between the ages of 8-13, along with teachers and activists, knocked over the wall and were met by about 40 armed police officers, some of whom fired tear gas canisters at the demonstrators.

Dozens of children were caught in choking clouds of tear gas before being forced to shelter on a pedestrian bridge over the main road to escape the noxious fumes.

Children screamed as police officers in riot gear and waving truncheons pushed them back, some panicking as they sought water to wash their burning eyes.

At least five children received medical treatment while one policeman was wounded, seen with blood pouring down beneath his riot helmet.

A senior police officer at the demonstration, Mwangi Kuria, said that officers had been deployed to “safeguard the property”, adding that rocks had been thrown at his men.

“This is disputed land, but they should not use the children,” Kuria was quoted as saying.

Two activists were detained by police, Kenyan media said.

Macharia Njeru, chairman of the Independent Policing Oversight Authority, said the incident would be investigated.

“Tear-gassing children is inexcusable,” he said.

Renowned activist Boniface Mwangi said a “school playground is a necessity, not a privilege.”

Some of the children carried placards protesting against the alleged land grab.

“Kenya: the land of shameless grabbers,” read one, with others pleading for government to fight corruption.

“Shame!” read another. “Grabber, you won’t live forever.”

The incident sparked angry reactions on social media, including Twitter, where comments used the hashtag “£OccupyPlayGround”.

“Shame on the government for assaulting children with tear gas to protect the corrupt,” politician and former presidential hopeful Martha Karua said on Twitter.

Monday was the first school day since the wall was built during the holidays.

Nairobi, a city of more than three million people, is rapidly growing and land prices are rising at some of the fastest rates anywhere on the continent, according to real estate experts.

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