US diplomats in Turkey feared that a wave of arrests of senior military officers last year over an alleged plot to topple the country's Islamist-rooted government could trigger an "unpredictable military reaction", according to a leaked diplomatic cable.

About 200 active and retired Turkish military officers, including former chiefs of the air force and dozens of generals and admirals, went on trial in December on charges of "attempting to topple the government by force" as part of a plot dubbed Sledgehammer.

Turkey's military, which has overthrown three governments since 1960 and put pressure on an Islamist-led government to step down in 1997, has denied such a plot existed. But a cable sent from the US embassy in Ankara after the arrests last February reveals fears of a threat to Turkish democracy.

"Some knowledgeable embassy officers see this latest step as a more serious provocation that could trigger some type of unpredictable military reaction. We will see," stated the cable, which was classified as confidential by the-then US ambassador to Turkey, James Jeffrey.

The alleged plot, which first surfaced after a Turkish newspaper said it had discovered documents detailing plans to bomb two Istanbul mosques and provoke Greece into shooting down a Turkish plane, was similar to – and said to be linked with – a larger alleged conspiracy by secular civilian and military nationalists, dubbed Ergenekon.

Senior Turkish military figures have consistently claimed coups are a thing of the past in the country, but the US cable took a different view. "There is some fire behind the smoke," it stated at one point, as it sought to set out "the facts" on this whole Ergenekon set of events.

"The military obviously has plans to intervene if necessary in political affairs and can cite the 1982 constitution, endorsed by the population per referendum, which gives the military a key role in 'overseeing' democratic governments' adherence to Ataturkist principles – largely defined as by the military and its friends in the bureaucracy and judiciary."

The cable noted that the 47 retired and active-duty military officers detained on 22 February included Ergin Saygun, a former deputy chief of Turkish's defence forces, who was "very well-known to the US" as a co-ordinator with the Americans on anti-terrorism issues for many years. He had accompanied the Turkish prime minister, Recep Tayyip Erdogan, on a visit to Washington in November 2007.

It suggested the investigations were related to electoral politics – "albeit of a below the belt, contact sport variety" – and an attempt by Erdogan to repeat previous electoral successes, when he was said to have "played off the military's counter-productive threats".

But it added: "All this is exacerbated by the thuggish authoritarian behaviour of the police and judiciary (reflecting prevailing tendencies in this society, including in the military). In the US a prosecutor or detective would simply have visited the generals in question to post questions. Invites to the precinct, reading of rights, indictments, arrests, and detentions follow only after the amassing of evidence and clear indications of a case winnable in court.

"Not here. Anyone even suspected of 'having information' is hauled before the police (armed with automatic weapons), and humiliated before the press. "

The cable said most of those who had been "publicly humiliated" in this way in the past had been released eventually, but it concluded with an ominous warning: "Every day is a new one here, and no one can be certain where this whole choreography will fall out of whack. Then, look out."

The alleged plots

Sledgehammer was a supposed plot to provoke a military coup, allegedly based on plans to blow up mosques during Friday prayers, assassinate Christian and Jewish leaders and cause the destruction of a Turkish warplane in an incident that would be blamed on Greece. The army has said the scenarios were discussed but only as part of a planning exercise at a military seminar. Twenty military officers were charged after a series of arrests last February.

Ergenekon, named after a legendary valley believed to be the ancestral homeland of the Turkish people, was an alleged conspiracy by secular nationalists to bring down the government. More than 400 people have been put on trial in hearings that have lasted for over two years. The controversy first broke into the public domain in June 2007 when a cache of explosives was discovered and a number of former soldiers were detained. Two former generals and a senior journalist were among 20 arrested in July 2008 for "planning political disturbances and trying to organise a coup".

Critics claim the cases are built on flimsy evidence and are designed to silence opponents of the government.