British authorities entered the Bahraini embassy in London late Friday, arresting a protester who claimed he was beaten by embassy staff after clambering onto the roof.
Video footage posted on Twitter shows the man, named by fellow protesters as Moosa Abd Ali, scaling on the roof before disappearing from view.
Abd Ali earlier wrote on his Twitter page that he was demonstrating against the execution of two men, named by him as Shiite citizens Ali Al-Arab and Ahmed Al-Malali, on terror charges.
"Police were called to the Embassy Of The Kingdom Of Bahrain at 22:47hrs (2147 GMT) on Friday, 27 July following a report of a man on the roof of the building," a police statement said Saturday.
"Upon hearing a disturbance on the roof, officers entered the building and detained the man.
"The man... was arrested for trespass ... He is currently in police custody." Police said they followed firefighters into the west London building.
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Campaigners said the authorities broke down the door, although the London Fire Brigade would not confirm this account.
The footage also showed police demanding that embassy staff "get away from" the protester while he was on the roof.
Protest group The Bahrain Institute for Rights and Democracy (BIRD) claimed the protester was "pulled from the ledge of the roof by embassy staff...and assaulted by two embassy staff".
"He stated that he sustained injuries and bleeding during the assault," added BIRD director Sayed Ahmed Alwadaei.
Police said the man, who could be heard after he was detained shouting "Stop the killing in Bahrain", would receive a medical examination.
The convicts were put to death by firing squad on Saturday.
Last year's killing of Jamal Khashoggi, a writer and US resident who was strangled and dismembered after being lured into Saudi Arabia's Istanbul consulate sparked worldwide condemnation and a debate on the limits of diplomatic immunity.
Bahrain, a key US ally located between regional rivals Saudi Arabia and Iran, has been gripped by bouts of unrest since 2011, when authorities cracked down on Shiite-led protests demanding political reform.
On the eve of the executions, Amnesty International's Middle East research director Lynn Maalouf said: "If the Bahraini authorities go through with these executions it would be an utterly shameful show of contempt for human rights."