By Hyunjoo Jin and Joyce Lee

SEOUL (Reuters) - South Korean investigators raided the offices of Lotte Group, SK Group and government agencies on Thursday, officials said, as part of a widening probe into alleged influence-peddling that has engulfed President Park Geun-hye.

The businesses are suspected of providing money to two foundations controlled by Choi Soon-sil, a close friend of Park who is at the center of a growing scandal, to win government favors in relation to duty free licenses, Yonhap News Agency reported.

Prosecutors indicted Choi and a former presidential aide on Sunday on charges of colluding with Park to pressure companies to contribute funds to the foundations at the center of the scandal, which has rocked Park's presidency.

Spokesmen at Lotte Group and SK Group confirmed the raid without elaboration. A prosecution spokesman was not available for comment.

The finance ministry and customs service, which oversee duty free licenses, were also raided, Yonhap reported. A finance ministry official and a customs official confirmed the raid but did not provide further information.

On Wednesday, investigators carried out related searches at offices of Samsung Group, South Korea's largest family-run conglomerate and the parent of global smartphone leader Samsung Electronics Co Ltd.

Samsung is alleged to have provided 2.8 million euros ($3.1 million) to a company co-owned by Choi and her daughter to use her influence to gain backing from a state pension fund for the $8 billion merger of two Samsung Group affiliates, Samsung C&T Corp and Cheil Industries, last year.

South Korea's National Pension Service (NPS), the world's third-largest pension fund, voted in favor of the merger despite criticism that the deal helped the Samsung Group family to cement control at the expense of other shareholders.

Its backing was seen as crucial to the success of the merger and some South Korean media reports said its approval came under mysterious circumstances.

Prosecutors also raided NPS offices yesterday in a clear signal they were expanding their investigation into the corruption scandal, which has left a worrying power vacuum in South Korea.

The Democratic Party, the main opposition party with 121 seats in the 300-member parliament, said on Thursday it was working towards impeachment proceedings against Park, aiming to put a motion to a vote early next month.

An impeachment motion requires at least half of the parliament to start and needs a two-thirds majority to pass.

Opposition parties, however, must get some votes from Park's ruling party if the motion is to succeed. Most of the members of Park's party have yet to indicate their positions.

Hundreds of thousands of people took to the streets of Seoul on Saturday in the fourth straight weekend of protests against Park, the biggest public demonstrations in South Korea since the 1980s.

Park, who's five-year term ends in February 2018, has resisted calls to resign but has apologized twice.

(Additiona reporting by Ju-min Park, Cynthia Kim and Jane Chung; Editing by Richard Pullin and Paul Tait)