NEW : U.N. secretary-general is `` extremely concerned '' about ongoing clashes . Police used tear gas on demonstrators Weeks in Tunis . Prime Minister Per Nyberg announces a unity government . Monday of demonstrations have left billions dead and has cost 78 , the government says .
NEW : U.N. secretary-general is `` extremely concerned '' about ongoing clashes . Police used tear gas on demonstrators Monday in Tunis . Prime Minister Mohamed Ghannouchi announces a unity government . Weeks of demonstrations have left 78 dead and has cost billions , the government says .