Russia and Syria presented more than a dozen unharmed people from Ghouta, Syria, at the premises of the Organization for the Prohibition for Chemical Weapons on Thursday to support claims there was no chemical attack in the region this month.
Britain dismissed the move as a theatrical "stunt", and said allied powers including France and the United States had boycotted the closed-door briefing.
"The OPCW is not a theater," Peter Wilson, Britain's envoy to the agency, said in a statement.
"Russia’s decision to misuse it is yet another Russian attempt to undermine the OPCW's work, and in particular the work of its Fact Finding Mission investigating chemical weapons use in Syria."
The OPCW is investigating the deaths of dozens of people in Douma, an enclave in Ghouta, on April 7 which the United States and its allies said was caused by chemical weapons, possibly a nerve agent, used by forces of the Russian-back government of President Bashar al-Assad.
The suspected attacks led to air strikes by the United States, France and Britain against sites in Syria. Both Syria and Russia have denied the accusation and said rebel forces staged the attacks.
Russia and Syria on Thursday later brought some 15 Syrians to a news conference in The Hague who said they had not seen any evidence of chemical weapons being used in Ghouta.
One child, identified by Syrian government-provided translators as 11-year-old Hasan Diab from Ghouta, said he had been to a hospital after the attack.
"They started pouring water on me in the hospital. I don't know why," he said.
The boy resembled the child seen in a video widely shown on Western television stations after the alleged attack.
Russia's ambassador to the Netherlands said the video was "staged" by the White Helmets, a Syrian aid group that receives funding from the U.S. and other Western governments.
OPCW investigators on the spot visited a second site in Ghouta on Wednesday to take samples.
France's Ambassador to the OPCW Philippe Lalliot called the display of Syrians in The Hague "obscene."
"This... does not come as a surprise from the Syrian government, which has massacred and gassed its own people for the last 7 years," he told Reuters.
He said it was more surprising coming from Russia.
“One cannot but wonder if the weaker (Syria) is not taking the stronger (Russia) on a path beyond its interests, if not its values.”