"Resign and prison," chanted anti-government protesters as public anger erupted on the streets of Sofia at the weekend.

Thousands marched in the Bulgarian capital on Sunday, blocking traffic for hours as the increasingly violent demonstrations entered a fourth day.

It was sparked by government-ordered police raids on the office of the Socialist-backed president, Rumen Radev, who is a critic of Prime Minister Boyko Borissov. Critics said this was an attack on the presidency and was at odds with the fact that powerful oligarchs connected with the government remain untouchable.

On Saturday, hundreds of people invaded a small beach on the Black Sea to protest against the perceived impunity of high-ranking officials who have broken the law.

The Black Sea coast is public property but access to the beach has been cut-off as it is near an estate used by the leader of a small liberal party that backs Borissov.

Radev has added his voice to calls for the government to resign, saying Bulgarians are fed up with what he called its "mafia-type" structure.

Different political factions have joined forces to oppose the ruling party, which they say uses EU funds to create a so-called "empire of power."

"It's kind of a ragtag group of people who are coming together, trying to put aside their differences although they've been implacable enemies for many years on the political front," said freelance cameraman Dimiter Kenarov, who was filming the demonstrations.

The political scandals of recent months come amid growing public anger over the government's handling of the coronavirus pandemic, with cases spiking after restrictions were relaxed.

Tensions were heightened by groups of counter protesters, who government critics claim were bussed in from the countryside by the government.

"They were put right across from each other, basically 50 meters from each other, which creates great danger for violence, it increases the rancour and hate between people," Kenarov said.

The buses can be seen on video being attacked by another group of protesters.

Kenarov said there are certain groups among the anti-government protesters whose intention is to provoke mayhem.

"I could hear them actually planning on how to attack the police, they split off from the main protest, and went to attack those buses and to attack the police, so that was sort of a goal to create mayhem, to create violence, but the vast majority of protesters have been very friendly," he said.

There are also fears that reports of police violence could escalate tensions.

"There are two or three cases of people beaten heavily after they were arrested, so we could expect some escalation in the next days," said Georgi Tsvetkov, a professor of defense and security at CSDM in Sofia.

In a Facebook Live at the weekend, Borissov did not address the corruption allegations, but accused Radev of acting irresponsibly and said the president's Socialist Party allies would "break the state."

He said he has no plans to step down because only his leadership has the support of the EU and provides the stability Bulgaria needs in this tumultuous time.

He also appealed to protesters to respect the police and refrain from violence.