A Manhattan man nursing a grudge against the government was indicted on arson charges Thursday for torching four cars and the lobby of an upper West Side apartment building, prosecutors said.

Christopher Buxo, 54, who left crazed notes at the scenes of the February fires, was carrying a kitchen knife, a loaded pistol and gasoline when he was busted Feb. 19, prosecutors said.

After the court entered a plea of not guilty for Buxo, prosecutors said they would not oppose a report that found him mentally incompetent to stand trial. Buxo was turned over to the city Health Department.

A joint FDNY-NYPD task force busted the firebug after investigators connected a Feb. 4 blaze that consumed a Ford Winstar on Central Park West and a fire a week later that destroyed a Volvo on W.87thSt.

While executing a search warrant at Buxo's Washington Heights apartment, authorities found a sawed-off shotgun, 200 rounds of ammunition, gasoline and a day planner noting the date and time of each of the fires, prosecutors said.