TIRANA, Jan 8 (Reuters) - Police fired tear gas and water cannon in Tirana on Saturday as stone-throwing protesters stormed into the headquarters of Albania"s opposition Democratic Party in a deepening power struggle between party rivals. At least one police officer and one protester were injured and dozens of protesters were arrested, police said. The protests were dispersed in the afternoon. The protesters were supporters of former president and prime minister Sali Berisha, who was thrown out of the party last year after Washington banned him from entering the United States over alleged corruption. read more Berisha, who denies wrongdoing, has since mounted a leadership challenge against party leader Lulzim Basha. Last month, Berisha called a party assembly and announced himself as leader. During Saturday"s unrest, Berisha supporters used hammers to smash open newly installed metal security doors at the offices and threw up ladders in a bid to reach the second floor. Police said in a statement they were forced to intervene after "a group of lawmakers inside the Democratic Party requested police help because lives were in danger". Local media said people inside the building had sought to keep the protesters out by spraying fire extinguishers before police arrived. "The battle will continue … we consider the party building as our home and we will liberate our home," said Berisha, speaking just after he was stopped from approaching the building again by police using paper spray against him and his supporters. The EU office in Tirana called for calm and restraint: "There must be no room for violence in politics," it said. The U.S. ambassador in Albania, Yuri Kim, said Washington was deeply concerned about recent tension surrounding the Democratic Party. "Those inciting violence or undermining the rule of law will be held accountable," Kim said in a tweet.