Ankara – Turkish prosecutors launched an investigation on whether a protest sign held at a Swiss rally and calling for the killing of Turkish President Tayyip Erdogan violated laws against inciting violence, police said on Monday.

The Swiss have also launched a similar probe.

On Saturday several thousand people, including Kurdish protesters, joined a rally in the Swiss capital Bern 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.

People demonstrating in Bern on Saturday held up a sign reading “Kill Erdogan with his own weapons” and pictured a pistol aimed at his head.

Turkey’s foreign ministry summoned Swiss diplomats in Ankara, demanding legal action against people at the rally.

Erdogan himself, who has accused Germany and the Netherlands of Nazi-style tactics for preventing rallies supporting his proposed new powers said Switzerland had gone even further.

In a speech in Istanbul on Sunday, he once again lashed out at European countries, saying that through their media they were campaigning for a “No” vote.

“Switzerland took it a step further. Their leftist parties and the terrorists … have come together and carried out a march. In the Swiss parliament, they hang my picture with a gun to my head. The Swiss parliament remains silent in the face of this,” Erdogan said.

A spokesman for the Swiss foreign ministry declined to comment on Erdogan’s accusations.

Bern police spokesman Dominik Jaeggi said prosecutors would determine whether Swiss law was violated by the sign, which police had not confiscated at the event.

“Police did not actively intervene in the demonstration,” Jaeggi said. “We confirmed the existence of the protest banner.”

The Turkish foreign ministry says the protest was organized by the outlawed Kurdistan Workers Party (PKK), designated a terrorist organization by Turkey, the European Union and the United States, but not by Switzerland.

An email announcing Saturday’s demonstration listed numerous groups supporting the event, including the Social Democratic Party, Switzerland’s second-biggest political party, as well as the Kurdish-Turkish-Swiss Cultural Association – but not the PKK.

A spokesman for the Social Democrats distanced the party from the sign, saying a splinter group not associated with the organizers had displayed it.

Secretary General Regula Tschanz of the Swiss Green party, which helped organize the demonstration with pro-Kurdish advocacy groups, insisted organizers had nothing to do with the gun image. Tschanz said the rally was meant to support peace and dialogue, and called the banner “counterproductive.”

“We are 100 percent against the content of that image,” Tschanz said, adding that she had no idea who was behind it.