By Humeyra Pamuk and Daren Butler
ISTANBUL (Reuters) - Turkish police detained the editor and senior staff of a leading opposition newspaper on Monday over its alleged support for a failed coup in July, in a move described by a top EU politician as the crossing of a red line against freedom of expression.
Updating earlier information on its website, Cumhuriyet newspaper said 11 staff including the editor were being held by authorities, and arrest warrants had been issued for five more.
Turkey's crackdown since rogue soldiers tried to seize power on July 15 has alarmed Western allies and rights groups, who fear President Tayyip Erdogan is using the coup attempt to crush dissent. More than 110,000 people have been sacked or suspended and 37,000 arrested over the past three and a half months.
The latest detentions came a day after 10,000 more civil servants were dismissed and 15 more media outlets shut down.
The Istanbul prosecutor's office said the staff at the paper, one of few media outlets still critical of Erdogan, were suspected of committing crimes on behalf of Kurdish militants and the network of Fethullah Gulen, a U.S.-based cleric. Turkey accuses Gulen of orchestrating the coup attempt, in which he denies any involvement.
"An investigation was launched... due to allegations and assessments that shortly before the attempted coup, material was published justifying the coup," the prosecutor's office said.
Cumhuriyet said several of its staff had their laptops seized from their homes. Footage showed one writer, Aydin Engin, 75, being ushered by plain clothes police into a hospital for medical checks.
Asked by reporters to comment on his detention, Engin said: "I work for Cumhuriyet, isn't that enough?"
Another veteran journalist, Kadri Gursel, who began writing for Cumhuriyet in May, said on Twitter that his house was being searched and that there was an arrest warrant for him.
Several hundred people gathered in front of Cumhuriyet's Istanbul offices in support of the paper, chanting and holding banners that said "Journalism is not a crime" and "Sharp pens will tear through the dark".
European Parliament President Martin Schulz wrote on Twitter that the detentions marked the crossing of 'yet another red-line' against freedom of expression in Turkey. "The ongoing massive purge seems motivated by political considerations, rather than legal and security rationale," he said.
The government has said its measures are justified by the threat posed to the state by the coup attempt, in which more than 240 people were killed.
A court on Sunday also jailed, pending trial, the co-mayors of the largely Kurdish city of Diyarbakir. The head of the pro-Kurdish Peoples' Democratic Party (HDP) called on opposition groups to stand together against a "tyrannical mentality".
"We are facing a new phase in the coordinated oppression managed by the AKP headquarters to ensure no opposition remains," Selahattin Demirtas told reporters. The AKP is the governing party.
Before turning himself in, veteran cartoonist Musa Kart told reporters outside Cumhuriyet's offices that such means of pressure were not going to succeed in frightening people.
"This is a comical situation," he said. "It is not possible for people with a conscience to accept this. You can't explain this to the world. I am being detained solely for drawing caricatures."
Cumhuriyet's previous editor, Can Dundar, was jailed last year for publishing state secrets involving Turkey's support for Syrian rebels. The case sparked censure from rights groups and Western governments worried about worsening human rights in Turkey under Erdogan.
Cumhuriyet said Dundar, who was freed in February and is now abroad, was one of those facing arrest.
"They are attacking 'the last bastion'," Dundar wrote on Twitter as news of the operation emerged. A month after the failed coup, Dundar told Reuters he feared the government would attempt to link him to the putsch.
Opposition groups say the purges are being used to silence all dissent in Turkey, a NATO member which aspires to membership of the European Union.
Since the attempted coup, 170 newspapers, magazines, television stations and news agencies have been shut down, leaving 2,500 journalists unemployed, Turkey's journalists' association said in a statement protesting the detentions.
"This operation is a new coup against freedom of expression and of the press," it said, adding that 105 journalists were in jail pending trial and the press cards of 777 journalists had been canceled.
(Writing by Daren Butler; Editing by Nick Tattersall and Mark Trevelyan)