Turkish police on Tuesday staged a major swoop on the Ankara-based offices of a media group critical of President Recep Tayyip Erdogan and close to his political nemesis, the US-based Muslim cleric Fethullah Gulen.

The raids sparked fresh concern about deteriorating press freedoms on Turkey, which is gearing up for a November legislative election, its second in five months.

The crackdown saw masked police breaking into and searching a number of offices belonging to Koza-Ipek Media, which owns several newspapers and two television channels, the Dogan news agency reported.

Police entered 23 offices as well as Ipek University in Ankara belonging to its parent company Koza Ipek Holding "as part of a terrorist investigation into Fethullah Gulen", the state-run Anatolia news agency reported.

It said the conglomerate's chief executive, Akin Ipek, was overseas.

Ipek Media Group owns the Turkish dailies Bugun and Millet, the television channels stations Bugun TV and Kanalturk and the website BGNNews.com.

It was the latest move targeting supporters of the powerful "Hizmet" (Service) movement headed by Gulen, a former ally-turned-foe whom Erdogan accuses of trying to create a "parallel state" with the aim of overthrowing his government.

As Erdogan and his ruling Justice and Development Party (AKP) prepare for snap elections on November 1 under the shadow of a major offensive against Kurdish militants, opposition journalists have complained of a major squeeze on all critical media.

The swoop came a day after a court in Turkey's Kurdish-dominated southeast ordered two British journalists to be held on "terror charges".

The two work for the US-based media outlet Vice News in a case that has sparked fresh concern about press freedom.

- 'Shameful!' -

Erhan Basyurt, editor-in-chief of the Bugun daily denounced the raids on Twitter.
"A police operation that aims to silence our group has started. Police raided our office. How shameful!" he wrote.

Several days ago, Fuat Avni, a mysterious Twitter user who has previously released sensitive claims about Erdogan's entourage, had warned of an imminent police raid targeting opposition media as the government wages a major campaign against Kurdish rebels.

Besides the Ipek group, Avni said two other opposition dailies, Sozcu and Tarak, were in the firing line as well as media owned by Dogan Group, the biggest independent press group in the country.

The government has repeatedly tried and failed to block Avni's presence on Twitter but he simply moves to another account.

"Erdogan has ordered that critical media be silenced," wrote Avni.

For several years Gulen was seen as a close ally of Ergodan and his Islamic-rooted AKP.

But in 2013 the authorities blamed Gulen for corruption allegations that rocked Erdogan and the ruling elite, and launched an all-out war against him and his supporters.

Gulen, who left for the US in 1999 to escape charges of anti-secular activities by the government at the time, has denied being behind the graft allegations against Erdogan.

The movement brings together a wide range of interests, from schools and media to finance, and is believed to be have the support of millions of Turks.

- 'Can't hide the truth' -

In December, police arrested dozens of people in raids on media outlets with ties to Gulen. The pro-Gulen Samanyolu TV (STV) chief Hidayet Karaca has been remanded in custody on charges of leading a terrorist group.

There has been growing concern about deteriorating press freedoms on Turkey and in particular over the numbers of journalists facing legal proceedings on accusations of insulting Erdogan.

Erdogan caused outrage in the run-up to June 7 elections by saying Cumhuriyet newspaper editor-in-chief Can Dundar would "pay a heavy price" over a front-page story which it said proved Turkey had sent arms to Islamist rebels in Syria.

"You can't hide the truth even if you silence all the newspapers," Dundar wrote on Twitter after the raids.

The front page of the anti-government Sozcu daily carried blank columns signed by its columnists in protest at the "unprecedented pressure on the media, the likes of which was not seen even during Turkey's coup era."

Meanwhile the mainstream Milliyet daily has in the last week dismissed two high-profile columnists -- Kadri Gursel and Mehves Evin -- after they wrote pieces critical of the government.