Turkish authorities arrested more than 1,000 people on Wednesday they said had secretly infiltrated police forces across the country on behalf of a U.S.-based cleric blamed by the government for a failed coup attempt last July.

The nationwide sweep was one of the largest operations in months against suspected supporters of the cleric, Fethullah Gulen, a former ally of President Tayyip Erdogan who is now accused by the government of trying to topple him by force.

Interior Minister Suleyman Soylu said the overnight crackdown targeted a Gulen network "that infiltrated our police force, called 'secret imams'.

"One thousand and nine secret imams have been detained so far in 72 provinces, and the operation is ongoing," he told reporters in Ankara.

In the aftermath of the failed July coup, authorities arrested 40,000 people and sacked or suspended 120,000 from a wide range of professions including soldiers, police, teachers and public servants, over alleged links with terrorist groups.

The latest detentions came 10 days after voters narrowly backed plans to expand Erdogan's already wide powers in a referendum which opposition parties and European election observers said was marred by irregularities.

The referendum bitterly divided Turkey. Erdogan's critics fear further drift into authoritarianism, with a leader they see as bent on eroding modern Turkey's democracy and secular foundations.

Erdogan argues that strengthening the presidency will avert instability associated with coalition governments, at a time when Turkey faces multiple challenges including security threats from Islamist and Kurdish militants.

"In Turkey, there was an attempted coup with a goal of toppling the government and destroying the state," he told Reuters in an interview late on Tuesday.

"We are trying to cleanse members of FETO inside the armed forces, inside the judiciary and inside the police," he said, using an acronym for the label, Gulenist Terrorist Organisation, which the government has given to Gulen's supporters.

The president compared the struggle against Gulen with the state's battle against Islamic State and Kurdish PKK militants, who are designated terrorist organisations by Turkey, the European Union and the United States.

"We are going to keep up the fight in terms of democracy, fundamental rights and liberties, but at the same time we are going to keep up the fight against PKK, FETO and other terrorist organizations such as Daesh (Islamic State)," he said. "We will continue down this path in a very committed fashion."

Mass detentions immediately after the attempted coup were supported by many Turks, who agreed with Erdogan when he blamed Gulen for orchestrating the putsch which killed 240 people, mostly civilians. But criticism mounted as the arrests widened.

Many relatives of those detained or sacked since July say they have nothing to do with the armed attempt to overthrow the government, and are victims of a purge designed to solidify Erdogan's control.