Telegram founder Pavel Durov has criticised France's investigation into illegal content on his messaging app, saying authorities were "struggling" to find evidence of wrongdoing a year after his high-profile arrest.

"One year ago, the French police detained me for four days because some people I'd never heard of used Telegram to coordinate crimes," Durov wrote on his Telegram channel Sunday evening.

"A year later, the 'criminal investigation' against me is still struggling to find anything that I or Telegram did wrong," he said.