Russia has produced what it says are witnesses from Douma in Syria, aiming to support its claim that an alleged chemical attack there was staged.
The witnesses spoke at a press conference at the headquarters of the Organisation for the Prohibition for Chemical Weapons, which is investigating the alleged attack.
The UK, US and France boycotted the event, which a UK official dismissed as a "despicable" stunt.
"The OPCW is not a theatre," said Peter Wilson, UK envoy to the watchdog.
"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," he said.
Russian officials produced an 11-year-old boy, Hassan Diab, who was filmed by activists from the White Helmets organisation at a hospital in Douma in the aftermath of the attack, in a video which was widely shared on social media.
Alongside his father at the Russian press conference, Diab said that he had heard cries of an attack and run to the hospital where he was doused with water. His father said at the press conference that he did not believe there was a chemical attack.
Russia also produced a worker who said he was in the hospital that day, and an emergency doctor, who both said the symptoms seen in patients that day were caused by smoke and dust from ordinary bombing.
But the footage of Diab did not show any symptoms of chemical poisoning, whereas other footage said to have been filmed at the site of the attack appears to show many dead children and adults with symptoms consistent with a chemical attack - including foaming at the mouth and corneal burns.
Four doctors and medical staff working in Douma that day told the BBC over several interviews that they treated patients with symptoms consistent with exposure to chlorine and possibly nerve agents.
Images widely shared by Syrian opposition groups and medics in the wake of the attack were too graphic to be published by the BBC and other news outlets.
OPCW inspectors are currently examining the site of the alleged attack in Douma. Western powers have accused Syria and Russia of delaying their deployment to the town. Experts said evidence could have been washed away or destroyed during that time.
The UN concluded last year that the Syrian government, led by President Bashar al-Assad, was responsible for the use of nerve agent Sarin in an attack last April that killed more than 80 people.
Sarin was also found by UN inspectors at the site of a 2013 attack in Ghouta which killed hundreds of people.
The Syrian government has denied involvement in any of the three alleged chemical attacks.
UK and French officials roundly criticised Russia's press conference.
"The director general (of the OPCW) has asked states to supply information about the Douma attack to his fact finding mission. Russia and Syria should do so, instead of waging a propaganda campaign of misinformation," said Mr Wilson.
The French ambassador to the Netherlands, Philippe Lalliot, said: "This obscene masquerade does not come as a surprise from the Syrian government, which has massacred and gassed its own people for the last seven years."
Correction 3 August 2018: An earlier version of this article reported that OPCW inspectors had been delayed access to the site of the alleged attack in Douma for two weeks by Russia. This has since been amended to clarify that western powers made this allegation and did not give a timeframe.