Albania's center-right opposition has called for further protests as it continues a months-long effort to bring about the resignation of the left-wing government and force early parliamentary elections.
The Democratic Party said after a meeting of its leaders on May 12 that it will hold another protest on May 13.
The move comes after thousands of demonstrators gathered early on May 11 in front of the main government building at Tirana's Martyrs of the Nation Boulevard before moving on to other sites.
Protesters threw flares, firecrackers, Molotov cocktails, and other items as security personnel used tear gas when some in the crowd broke through a police cordon and attempted to storm the government building.
Protesters did not enter the site, and clashes were later reported near the parliament building.
"Barbarous violence against hundreds of protesters...will get [on May 13] the proportional response from the united and determined people," Lulzim Basha, leader of the Democratic Party, told hundreds of supporters at the Tirana police department on May 12.
The opposition has alleged fraud in the 2017 parliamentary elections, organized protests, and cut its ties to parliament to force early polls. The Socialist government of Prime Minister Edi Rama has dismissed the allegations.
The opposition also accuses Rama's cabinet of having links to organized crime, which the government denies.
The protests are taking place weeks ahead of an expected decision by European Union members whether to approve opening accession talks with Albania and neighboring North Macedonia.
The EU and the United States have expressed support for the government and have urged the opposition to return to parliament and take part in local elections on June 30.
Both Rama and Basha have backed the moves to join the EU, but the opposition leader has said the Socialists' alleged corruption could prevent the small Western Balkan country from reaching its membership goal.
Protesters are "determined to keep waging a bigger and more resolute battle as long as the government was keeping Albania apart from Europe," he said.
"We are here with a mission, to liberate Albania from crime and corruption, to make Albania like the rest of Europe," Basha told supporters at a May 11 rally in central Tirana. "I call on our European and international friends not to punish Albania."
Ambulances were seen taking injured people away from the protests, but there was no immediate information on numbers of casualties.
Interior Minister Sander Lleshaj said 13 policemen were injured.
President Ilir Meta called on protesters "to avoid acts of violence and confrontation," while Rama denounced the violence and expressed support for police forces.
The U.S. Embassy in Tirana condemned protesters' violence and called for restraint by all sides.
"Protest leaders have a responsibility to encourage calm. We call on all parties to show restraint," a statement said.
The Organization for Security and Cooperation in Europe said that "violence is not the answer. Also, heavy response to violence will not help."