Dubai police confirmed a 55-year-old British woman was being tried for defamation under the UAE's cybercrime law.
Police said the woman had never been held in custody, but had confessed to making the statements and refused attempts to settle out of court.
While police would not disclose the woman's identity, the London-based group Detained in Dubai identified her as Laleh Sharavesh,
saying she had described her ex-husband's new wife as "a horse".
Dubai police said that following a complaint by a Portugese man and his Tunisian wife in 2017, a warrant had been issued requiring the woman to present herself to the police.
"Upon her return to the country on 10 March 2019, the outstanding warrant prompted the immigration authorities at the airport to bring the matter to the British woman's attention," the police said in a statement.
"She was requested to attend the nearest police station." Britain said it was in contact with the UAE about the case and offering help to the Sharavesh.
"We are offering consular assistance to Sharavesh. We're very concerned about the situation," British Foreign Secretary Jeremy Hunt told reporters in Luxembourg.
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"Our diplomats in the UAE have enormous experience in dealing with consular cases ... so she is getting the best possible service" from the British authorities, he added.
The Detained in Dubai group said Sharavesh's Portuguese husband of 18 years divorced her to marry a Tunisian woman in Dubai.
The man died last month and Sharavesh flew to Dubai with their daughter for his funeral, according to the group.
While their teenage daughter was allowed to fly back, Sharavesh was not permitted to leave the country.
The group said the charges are based on two comments Sharavesh posted under her ex-husband's new wedding pictures on Facebook in Farsi.
"I hope you go under the ground you idiot. Damn you. You left me for this horse," she allegedly wrote. "You married a horse you idiot".
Sharavesh's attorney could not be reached for comment.
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