His friend and fellow campaigner Mark Hirst said Mr Murray was detained under terrorism laws on returning from a protest in Iceland, and has since been released.
The detention was also reported by the Campaign Against Antisemitism, which said it had alerted police to a social media post by Mr Murray, 65.
“I have always viscerally opposed war. I have dedicated my life to conflict resolution and reconciliation,” Mr Murray wrote on Sunday.
“But in the coming Gaza genocide, every act of armed resistance by Hamas and Hezbollah will have my support. If that is a crime, send me back to jail.”
Mr Murray is a former British ambassador to Uzbekistan who fell out with his bosses after going public with allegations over human rights in the country.
In 2021 he was sentenced to eight months in prison on contempt of court charges, linked to posts about the trial of former Scottish first minister Alex Salmond.
A supporter of Julian Assange, he was involved in a dispute with the Ministry of Justice over whether he could attend the WikiLeaks founder's wedding to lawyer Stella Morris in Belmarsh Prison in London.