Turkey has launched an investigation into the football club Galatasaray after fans at a match held up a giant image of Sylvester Stallone and a slogan that government supporters said evoked the words of an exiled cleric accused of masterminding a coup attempt last year.

During their team’s goalless draw with archrivals Fenerbahçe in an ill-tempered Istanbul derby on Sunday, fans displayed a poster of Stallone in the film Rocky with the words: “They look big because you are on your knees, stand up.”

A pro-government daily said the caption evoked the words of Fethullah Gülen, the US-based cleric widely believed in Turkey to have orchestrated last year’s coup attempt, who closed a speech with a poem featuring the line “Rise up Sakarya,” referring to a province in Turkey.

Reuters said the prime minister, Binali Yıldırım, had ordered an investigation into the incident.

Galatasaray described the accusations as “pathetic” and the work of “puppets, pawns and social media trolls”. It said on its website: “We will use all our legal rights against any person or social media account that puts the name of Galatasaray beside the leader of an odious terrorist organisation.”

Tens of thousands of people in Turkey have been dismissed from their jobs or detained over alleged links to Gülen’s movement. Critics say the purges have gone beyond the perpetrators to crush all forms of dissent against the president, Recep Tayyip Erdoğan.

The pattern of media vilification of opponents has been repeated often in Turkey in the months since the coup attempt. Pro-government media, which now forms the large majority of the country’s local press, have often played the role of instigator, with media campaigns often followed by the opening of new investigations.