The Russian opposition leader Alexei Navalny has been freed after three weeks behind bars for organising anti-Kremlin protests. It was his second spell in detention in as many months.

The 42-year-old activist left a detention centre in the south of Moscow in the early hours of Sunday morning and went to a waiting car.

“Over the 50 days I have been in jail we have seen yet more evidence that this regime is in complete decline,” he said, referring to recent embarrassments to Moscow’s intelligence services abroad and the launch failure of a manned Russian space rocket.

“If anyone thinks that with arrests ... they can scare or stop us, that is clearly not the case.”

Authorities have turned up the heat on Vladimir Putin’s top critic since the Russian president’s approval ratings took a beating over deeply unpopular pension reforms.

Navalny finished a 30-day sentence in September for organising a rally at the start of the year but was arrested as soon as he was released to face further charges over another protest.

The latter demonstration, against the raising of the retirement age, was timed to coincide with regional elections in September.

The Kremlin suffered rare defeats in those polls, with voters rejecting candidates from the ruling United Russia party in at least two regions.

A run-off in the far-eastern Primorsky Krai will be held again in two months after accusations of vote-rigging in favour of the Moscow-backed candidate led to protests.

The Navalny ally Leonid Volkov said at the time of the latest arrest that the Kremlin had “to take it out on someone because of all their defeats and failures of the last weeks”.

Supporters fear that the two consecutive administrative cases mean the authorities may be getting ready to open a criminal case against Navalny. In that case he could face a lengthy prison term.

Amnesty International described Navalny as a prisoner of conscience and said he had committed no crime.