Seoul Mayor Park Won-soon joined the main opposition Democratic United Party Thursday, a shot in the arm for the liberals as they seek to prevail in upcoming parliamentary polls.

Park, a political neophyte, defeated a high-profile ruling party lawmaker in the by-election last October with the help of the liberal camp.

Park's mayoral bid was a catalyst for consolidation efforts by the liberals, who went on to merge various liberal forces into a party.

"Our goal in 2012 isn't too different from the one we had for the mayoral by-election," Park said as he announced his decision. "The public will change the parliament and the president."

The move is a boost to the Democratic United Party as it is posed to face off against the conservative ruling Saenuri Party in the April parliamentary and the December presidential polls.

Park was formerly a lawyer and civic activist.