Is it ok in a European democracy to call for a president's murder?

Hundreds of supporters of the outlawed PKK were allowed by Swiss authorities to rally against Turkey's President Recep Tayyip Erdogan on Saturday just days after European countries stopped his ministers from addressing Turkish expatriates.

Nearly 250 people gathered in front of the parliament in Swiss capital Bern, holding provocative placards that read "Kill Erdogan," along with PKK posters and flags.

PKK is behind a decades-long insurgency that has killed thousands of Turkish citizens.

It has been designated as a terrorist organisation not just in Turkey but also the United States and the European Union.

Yet, its supporters regularly stage protests and host rallies in European capitals.

In Bern, police did not disperse people who chanted "dictator Erdogan", "terrorist Erdogan" slogans during the protest which was also supported by the country's Green Party.

Turkey's foreign ministry summoned Switzerland's deputy ambassador to Ankara on Saturday over the protest.

The rally happens ahead of the April 16 presidential referendum in Turkey that the government says will help the country's economic growth and stem the tide of terrorism.

The country will vote on whether to move from a parliamentary to a presidential system of government.

Last week, German officials also allowed PKK followers to march in the city of Frankfurt after barring Turkish ministers from mobilising Turkish voters in German cities.

Turkey's presidential spokesman Ibrahim Kalin accused some European countries of "attempting to interfere in the referendum that will take place on April 16 in Turkey and take up a position in favour of a 'No' vote after the protest in Germany."