Turkish police have stormed the offices of a leading opposition paper linked to President Recep Tayyip Erdogan’s arch foe, in the latest crackdown on government rivals.

Turkish police have stormed the offices of a leading opposition paper linked to President Recep Tayyip Erdogan's arch foe, in the latest crackdown on government rivals.

As riot police armed with water cannon massed outside, officers and lawyers searched the Istanbul offices of the Zaman newspaper late Wednesday, the paper said.

Zaman said the raid, which followed a similar operation against opposition television stations last month, was a "threat to the free media".

The targeted media outlets are linked to Fethullah Gulen, a US-based preacher who was once a key supporter of Erdogan's Justice and Development Party (AKP) but has now become the president's greatest enemy.

Police launched Wednesday's action after claims that Zaman was printing a "pirate" version of a newspaper belonging to the multi-billion dollar Koza-Ipek conglomerate, whose companies were seized last month as part of a probe into Gulen and his network.

Zaman media group lawyers issued a statement branding the operation illegal and "shameful" at a time when the world's eyes are on Turkey for the G20 summit of the world's most powerful leaders opening this weekend.

Rights groups and Turkey's Western allies have voiced alarm at what is seen as a deliberate campaign to silence government rivals and critical media.

In a hard-hitting report this week, the European Union highlighted "serious shortcomings" in Turkey that affected the independence of the judiciary and freedom of expression.

Erdogan, who has dominated the political scene in Turkey for well over a decade, is viewed as increasingly autocratic by opponents who fear his party's landslide election win on November 1 will only cement his grip on power.

Gulen, 74, who from his US base runs a network of religious schools, charities and companies, has been charged with "terrorism" and plotting to topple the government.

The government accuses his allies in the judiciary and police of launching a corruption probe into Erdogan and his inner circle in 2013 that seriously tarnished the reputation of the prime minister turned president.