Moscow: Fugitive former US government contractor Edward Snowden on Thursday slammed the arrest in London of WikiLeaks founder Julian Assange, calling it a "dark moment for press freedom."

"Images of Ecuador's ambassador inviting the UK's secret police into the embassy to drag a publisher of -- like it or not -- award-winning journalism out of the building are going to end up in the history books," the whistleblower said on Twitter.

"Assange's critics may cheer, but this is a dark moment for press freedom."