ANKARA (Reuters) - The Turkish foreign ministry summoned Switzerland’s ambassador in Ankara on Sunday over a protest in Bern that it said was organized by supporters of terrorist groups and targeted President Tayyip Erdogan, ministry sources said.

People hold banners and flags during a demonstration against Erdogan dictatorship and in favour of democracy in Turkey in Bern, Switzerland March 25, 2017. The banner reads "Say no to Erdogan" REUTERS/Ruben Sprich

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.

Turkish foreign ministry sources said the Swiss charge d’affaires had been summoned on Saturday because Swiss ambassador Walter Haffner was out of Ankara.

On Sunday, Turkey told Haffner it expected the Swiss authorities to bring those responsible for the protest to justice and to take precautions to ensure such incidents do not happen again, sources said.

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.

In a speech in Istanbul on Sunday, Erdogan 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.

Organizers and Bern police said the rally had passed off peacefully, though images from the demonstration showed a poster depicting Erdogan with a pistol pointing at his head.

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.