New Delhi, Dec 28 - The Congress is set to launch its yearlong 125th anniversary celebrations Monday and leaders including Prime Minister Manmohan Singh and party president Sonia Gandhi will address a rally in the national capital.

Sonia Gandhi will hoist the party flag at the Congress headquarters here. She will also lay a foundation stone for a new office of the party to be constructed at Rouse Avenue near Connaught Place, Delhi's business hub, party sources said.

The construction of the new office is expected to be completed by the end of next year. (IANS)