Community service pioneer Alan Khazei is scheduled to join the Massachusetts Senate race on Thursday, rounding out the Democratic field in the special election to succeed Edward M. Kennedy.

Khazei, co-founder of the City Year volunteer program, plans to announce his campaign at noon Thursday in Boston. It will follow a Wednesday tour of western Massachusetts to talk to community activists. Khazei has already pulled nomination papers and filed the federal paperwork for a Senate run.

Khazei will be the upstart candidate in a Democratic primary field that contains two seasoned politicians — Attorney General Martha Coakley and 8th District U.S. Rep. Michael E. Capuano — and a multimillionaire businessman — Stephen Pagliuca.