German leaders have condemned the “unacceptable” attempt by protesters to storm the Reichstag building during a mass rally against coronavirus restrictions.

Some of the protesters on Saturday carried the former imperial flag, which was used up until the end of the first world war.

“Flags from the Reich and far-right profanity in front of the German parliament are an unacceptable attack on the heart of our democracy,” the German president, Frank-Walter Steinmeier, said on Instagram. “We will never accept this.”

Police said about 38,000 people, double the number expected, had gathered in Berlin to protest against restrictions imposed to curb the spread of the coronavirus, such as the wearing of masks and social distancing.

Late on Saturday, several hundred people broke through barriers and a police cordon to climb the steps leading to the entrance. They were narrowly prevented from entering the building by police, who used pepper spray and arrested several people.

Police initially appeared overwhelmed, as only a handful of officers were there to block the crowd.

“We cannot be present everywhere and it is precisely these weaknesses in the deployment that were exploited, in this case to cross the security barriers in order to reach the steps of the Reichstag,” said a local police spokesman, Thilo Cablitz.

The Reichstag building, where German MPs meet, has a powerful symbolic role in the country. The domed building was burned down by the Nazis in 1933 in an act aimed at destroying what remained of German democracy between the two world wars.

Images of the incident shocked Germany.

The Reichstag is the “symbolic centre of our democracy”, the interior minister, Horst Seehofer, told Bild. “It is unacceptable to see extremists and troublemakers use it for their own ends.”

About 300 people were arrested in scuffles with police, in front of the Reichstag building and also outside the nearby Russian embassy, where protesters pelted police with bottles.

Berlin city authorities had initially decided not to allow the Saturday demonstration to go ahead, fearing the protesters would not socially distance or comply with face mask requirements.

The ban sparked outrage from organisers and their supporters who flooded social media with angry messages vowing to protest anyway, with some even calling for violence.

But on the eve of the protest, Berlin’s administrative court sided with the demonstrators, saying there was no indication organisers would “deliberately ignore” social distancing rules and endanger public health.