BELGRADE, Serbia (AP) — A Serbian opposition alliance was holding a gathering in a southern city when a group of young men wearing dark clothes and hoodies approached one of its leaders and brutally attacked him with brass knuckles.

The thugs beat up Serbian Left party leader Borko Stefanovic, smashing his head and briefly knocking him unconscious. Two others who were with him were also seriously injured before the attackers fled the scene in a car.

Images of Stefanovic's bloodied head and blood-stained clothes have sent shockwaves through the Balkan country, reminding many of the times during the autocratic rule of late strongman Slobodan Milosevic in the 1990s that was marked by similar ruthless attacks against opponents.

In response to the beating, thousands of people poured into the streets of the capital Belgrade for the past two weekends, braving snow and freezing temperatures to demand an end to political violence in protests dubbed "No More Bloody Shirts!"

The opposition leaders have pointed their finger at populist Serbian President Aleksandar Vucic, a reformed extreme nationalist who they say has been restoring Milosevic-style autocracy even as he says he wants to lead Serbia into the European Union.

With more protests planned for Saturday, the leaders are pledging to boycott any future elections in Serbia before the government creates free and fair conditions for all.

Vucic's center-right Serbian Progressive Party has near complete control over the security apparatus, judiciary and media, leaving very little voice for the opposition and their allegations of widespread corruption, cronyism and voter intimidation that have marked his six years in office.

Critics say that Vucic and his allies — very much like once Milosevic — have revived nationalistic rhetoric and hate speech, intolerance and official propaganda that regularly labels opponents "traitors," ''criminals" and "foreign spies."

Although Serbia is nominally a democratic society with political opposition and free elections, Vucic's firm grip on power has made it hard for opponents to make their voices heard or answer verbal outbursts against them.

Vucic has publicly condemned the attack on Stefanovic and police swiftly arrested the assailants, but opponents like Dragan Djilas say he has fostered an "atmosphere of violence" that made such an attack possible.

"They have drawn targets on our backs," said Djilas, a former Belgrade mayor who now heads an opposition alliance that is behind the protests in the capital, told The Associated Press in an interview.

"None of us have said Vucic himself was behind the attack" Djilas said. "But in the past six years, Serbia has become a land of hatred and aggression, of insecurity."

Vucic has repeatedly denied the allegations, dismissing them as an expression of opposition frustration with poor election results and lacking in public support. He insists that his government has made important strides toward pro-EU reform and economic recovery.

The country remains crucial for stability in the volatile Balkans and Western officials have hoped that prospects of EU integration would push it toward reconciliation and reform needed for social and economic recovery. Serbia's pro-Western opposition politicians, however, have alleged that EU officials have turned a blind eye toward Vucic's stifling of democratic freedoms as long as he maintains peace in the region.

The latest EU report on Serbia's progress in accession reform praised the country's economic changes and cooperation in tackling migration, but it stressed a lack of improvement in areas such as media freedoms and the prosecution of war crimes.

In November, the European Parliament passed a resolution urging Serbian authorities to step up reform efforts on the rule of law, intensify the fight against corruption and organized crime, and continue to reform the country's public administration to make it more politically neutral.

"They praise Serbia's European road, what European road?" Djilas asked. "Unfortunately, we are further away from Europe each day instead of moving closer."

For Srbijanka Turajlic, a respected former university professor who played a prominent role in the anti-Milosevic movement and remains active in politics, the situation in Serbia is "even worse" than it was under Milosevic.

"We no longer have any institutions, no protection of any kind, just an autocrat," she said in an interview.