ANKARA (Reuters) - The Turkish government has launched an investigation into a poster unfurled by fans of Istanbul’s Galatasaray soccer team that pro-Turkish media said was clearly linked to organizers of the 2016 attempted coup.

Just before Sunday’s match kicked between Galatasaray and local rivals Fenerbahce, the home fans opened a giant red and yellow poster behind the goal showing Sylvester Stallone’s famous fictional boxer “Rocky”, with the caption “Stand Up” and “They look big because you are kneeling.”

Pro-government Turkish media linked the poster and its caption to the network of U.S.-based cleric Fethullah Gulen, whom Ankara accuses of masterminding the July 2016 failed coup attempt.

Pro-government daily Takvim said the poster was a reference to a speech by Gulen, where he was quoted reading from a poem that ends with the words “Stand Up Sakarya”. Sakarya is a province in northwest Turkey.

Prime Minister Binali Yildirim on Monday ordered authorities to carry out an investigation into the poster, sources from his office said.

The game between Galatasaray and Fenerbahce, Turkey’s biggest fixture, ended in a 0-0 draw after a match that was marred by violence in which a referee was injured.

In a statement, Galatasaray said the accusations were “a pathetic attempt” to discredit the club, adding that the same poster was used at a match in May.

“We will use all our legal rights against any institution, person and social media account who tried to put the Galatasaray name next to that of the red-handed leader of the heinous terrorist organisation by using the choreography as an excuse,” the club said on its website.

Some 50,000 people have been jailed pending trial over suspected links to last year’s failed coup and more than 150,000 have been sacked or suspended from their jobs in the military, public and private sectors.

Rights groups and Turkey’s Western allies have said President Tayyip Erdogan is using the failed coup as a pretext to crush dissent, but the government says the measures are necessary to fight the threats it is facing.

Following the statement from Yildirim’s office, Galatasaray’s shares were down 4.89 percent as of 1535 GMT.

Current league leaders Galatasaray are Turkey’s most decorated soccer club, with 20 league championships, and became the first Turkish club to win a major European trophy when they won the 2000 UEFA Cup, the predecessor to the Europa league.