ISTANBUL (Reuters) - Turkey’s foreign ministry summoned Switzerland’s charge d’affairs in Ankara on Saturday to complain about a protest in Bern that it said supported terrorism and included a poster calling for the assassination of President Tayyip Erdogan.

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Earlier on Saturday several thousand people including Kurdish protesters joined a rally in the Swiss capital calling for a ‘No’ vote in Turkey’s April 16 referendum that could give sweeping powers to Erdogan under a constitutional overhaul.

The referendum issue has already badly strained relations between Turkey and several European countries, including Germany, after they banned Turkish ministers from campaigning on their territory for a ‘Yes’ vote in the referendum.

The Turkish foreign ministry said it expected Swiss authorities to launch a criminal investigation into Saturday’s demonstration in Bern.

“The judicial and administrative steps that will be taken by Swiss federal and local authorities will be closely followed by our ministry and our efforts on this will continue,” it said in a statement.

Turkish Foreign Minister Mevlut Cavusoglu also phoned his Swiss counterpart to express Ankara’s anger, the ministry said.

A spokesman for the Swiss foreign ministry confirmed Ankara had summoned its envoy and said the incident would be investigated.

“The competent authorities will have to check whether the organizers of the rally have violated the permit requirements or if there are other criminal offences,” he said.

Organizers and Bern police said the rally had passed peacefully.

Michael Sorg, a spokesman for Switzerland’s Social Democrats, one of the organizers of the rally, confirmed the anti-Erdogan poster.

“This was the only poster which fell below the limits of decency. All other posters were decent,” he said.

On Saturday, Erdogan also slammed Switzerland over the demonstration and said the crowd had included supporters of terrorist groups.

Referring to the poster of him with a gun pointed to his head, Erdogan said: “Could there be such a mentality, such an understanding?”

Ankara has accused some European countries of allowing ‘No’ supporters to campaign freely ahead of the Turkish referendum while deliberately banning rallies planned by the ‘Yes’ camp.

Germany and the Netherlands, both home to many expatriate Turks with the right to vote in the referendum, have said the decision to ban several planned rallies was taken on security grounds and was not politically motivated.