BELGRADE, Serbia (AP) — Thousands ofanti-government protesters marched again in Serbiaon Monday with baton-wielding riot police chasing them in downtown Belgrade, prompting Serbia's populistPresident Aleksandar Vucicto reiterate his pledge of a planned major crackdown against the demonstrators. The protest in the country's capital was peaceful until a group of young people, believed to be soccer hooligans, split from the crowd and hurled rocks at ruling party's downtown offices, breaking front windows. The riot police soon appeared at the scene on armored trucks, dispersing the crowd and triggering a panicky escape by the crowd. In a show of defiance,Vucic showed up at his party's damaged office, calling the protesters terrorists and saying that "soon, the citizens will be freed from this terror and evil." Vucic on Sunday announced tough measures against anti-government protesters following days ofriots in the streets throughout Serbiathat have challenged his increasingly autocratic rule in the Balkan country. On Monday, he reiterated his claims that months of persistent protests against his rule have been orchestrated in the West and aimed at destroying Serbia. "Our country is in grave danger, they have jeopardized all our values, normal life, each individual," Vucic said. The stern warnings came after days of clashes between the protesters on one side and police and Vucic's loyalists on the other. Angry protesters on Saturday eveningtorched Vucic's governing Serbian Progressive Party officesin a town in western Serbia. Vucic did not specify what the state response would be in the coming days. Scores of people have already been detained and injured in the past days while police have faced accusations ofexcessive force and arbitrary detentionsof protesters. The clashes last week marked a major escalation of violence following more than nine months of largely peaceful demonstrations that started after aconcrete canopy collapsed at a train stationin Serbia's north, killing 16 people. Many in Serbia blamed the tragedy on alleged widespread corruption in state-run infrastructure projects. Vucic has faced accusations ofstifling democratic freedomswhile allowing organized crime and corruption to flourish, which he denies. Serbia is formally seeking European Union membership, but Vucic has maintained strong ties with both Russia and China.