ISTANBUL (Reuters) - The Turkish authorities issued detention warrants on Friday (Oct 7) for 166 people, including police chiefs, the state-run Anadolu Agency reported, in an operation linked to July's attempted coup, which Ankara blames on followers of United States-based cleric Fethullah Gulen.

The investigation was initially focused on Istanbul police headquarters staff but spread across 35 provinces, targeting people who used a little-known smartphone messaging app known as ByLock, state-run Anadolu Agency said.

In the post-coup crackdown, some 32,000 people have been jailed pending trial and around 100,000 members of the security and civil services, university professors and others have been fired or suspended from work.

Turkey's Western allies in the European Union and Nato have voiced concerns that innocent people may be swept up in the investigations, which critics have described as a purge.

Turkey enforced emergency rule and began dismissing, suspending and arresting state officials after the coup attempt in which rogue troops commandeered warplanes to bomb parliament and used tanks to kill 240 people, many of whom were civilians.

Ankara says followers of Mr Gulen staged the coup to overthrow President Tayyip Erdogan and seize control of the country. Mr Gulen, a one-time ally of Mr Erdogan, denies any involvement.

A senior Turkish official said in August the country's intelligence agency had identified at least 56,000 operatives of Mr Gulen's network after it cracked the ByLock app, which the group began using in 2014.