Hundreds of protesters scuffled with riot police late on Tuesday as they marched through downtown Yerevan at the start of a nonstop demonstration organized by an Armenian opposition alliance seeking to topple President Serzh Sarkisian.

The New Armenian Public Salvation Front, a coalition of hardline opposition forces, staged the march after rallying supporters in Liberty Square. Addressing the crowd, its leaders renewed their calls for Sarkisian to call off Sunday’s referendum on his controversial constitutional changes and step down.

“The revolution has begun,” one of them, Zhirayr Sefilian, declared before chanting “Death to the regime!” from the podium.

Sefilian and Raffi Hovannisian, another opposition leader, claimed that Sarkisian’s resignation is imminent, urging the Armenian police not to carry out “illegal” government orders. “A new Armenia is born,” said Hovannisian.

The crowd was stopped and pushed back by riot police when it approached the headquarters of the ruling Republican Party of Armenia (HHK). Police officers at the scene did not allow Hovannisian to lay a bouquet of flowers at the entrance to the building, sparking angry reactions from the protesters.

“Our pressure on the regime will be peaceful,” Sefilian said there through a megaphone. “Nobody must doubt that the regime will cave in.”

“We won’t go back home until we get rid of this regime,” the outspoken opposition added, urging more Armenians to join the “civil disobedience” campaign.

The protesters again briefly clashed with security forces shortly afterwards when the latter refused to allow a New Armenia van carrying sound amplifiers to escort the procession.

The police issued a statement later in the evening condemning the “provocative actions” of the oppositionists and urging Hovannisian and Sefilian to “sober up.” “The organizers of the gathering will bear full responsibility for the consequences of such actions,” warned a police statement.

Hovannisian rejected the warning and accused the police of hampering the protest sanctioned by municipal authorities as he walked with the protesters back to Liberty Square.

Addressing the remaining protesters in the square shortly before midnight, Hovannisian announced that New Armenia is beginning an around-the-clock sit-in there and will hold more rallies on a daily basis.

The opposition bloc apparently planned to set up a tent camp in the square but refrained from doing so after failing to secure permission from the municipal administration.

Hovannisian’s Zharangutyun (Heritage) party is also affiliated with the No Front, a more broad-based opposition coalition also campaigning against Sarkisian’s constitutional reform. Levon Ter-Petrosian’s Armenian National Congress (HAK), the most important No Front member, has a less radical agenda, primarily seeking a popular vote against the proposed amendments which it believes would effectively extend Sarkisian’s rule beyond 2018.The HAK has downplayed its “tactical” differences with New Armenia.

No Front plans to hold its next rally in Liberty Square on Thursday. Addressing New Armenia supporters on Tuesday, Vladimir Karapetian, a senior HAK member, said the No Front rally “will only reinforce this movement.”