Police detained prominent Kremlin critic Alexei Navalny on arrival in Moscow on Sunday after he flew home to Russia from Germany for the first time since he was poisoned last summer, triggering a political clash with the West.

The move, which could see Navalny jailed for 3.5 years for allegedly flouting the terms of a suspended prison sentence, may reignite political pressure on the West to tighten sanctions on Russia, especially against an $11.6 billion project to build a natural gas pipeline from Russia to Germany.