In-house investigators from the National Intelligence Service (NIS) confirmed that the agency's cyber warfare unit organised and operated up to 30 teams for more than two years in the run-up to the 2012 elections, the agency said in a statement late Thursday.
They hired internet-savvy civilians and sought to sway voter opinions through postings on portals and Twitter.
"The teams were charged with spreading pro-government opinions and suppressing anti-government views, branding them as pro-North Korean forces' attempts to disturb state affairs", it said.
Moon won South Korea's presidential vote in May this year after Park was impeached and dismissed over corruption and abuse of power, and ordered an investigation.
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He has vowed to reform the NIS to prevent it meddling in elections and make it focus on collecting and analysing intelligence on North Korea and foreign affairs.
A spokesman for Park's party, now in opposition and renamed Liberty Korea, said Friday the inquiry was "politically motivated".
Former NIS chief Won Sei-Hoon is being tried for the second time for leading an online smear campaign against Moon, after his initial conviction was overturned on appeal.
But the NIS investigation results suggest the scale of the voter manipulation was far wider than previously thought.
The internal probe also found Won ordered the agency to muzzle the press, provide support for pro-government conservative civic groups and put some major opposition politicians under secret surveillance.