The ship sinking has become the top campaign issue, with the liberal opposition accusing President Lee Myung-bak of provoking tension after a decade of warming ties with reclusive and impoverished North Korea.

But polls showed Lee's uncompromising stand against the North has helped keep support ratings for him near the 50 percent mark in recent weeks.

Lee has vowed to hold North Korea accountable after a team of investigators found Pyongyang was behind the torpedo attack on the corvette Cheonan in March, killing 46 seamen.

North Korea has denied responsibility and accused Lee of fabricating the attack for political gain.

"We are not going to tolerate activities that harm public interest and undermine national integrity through the spread of false information," Seoul police commissioner Cho Hyun-oh said in a message to officers, vowing to punish offenders.

The crackdown on Internet rumor mongering is likely to resurrect a frequent opposition complaint that the government is trampling on democracy and free speech.

Police are on their highest security alert ahead of Wednesday's voting to choose nearly 4,000 mayors, governors and local representatives, seen as a gauge of support for Lee and his ruling Grand National Party.

Lee is trying to avoid a repeat of massive protests that consumed the first months of his term in 2008 when he failed to contain the spread of information on television and the Internet that experts said hugely exaggerated the health risks of eating US beef amid a mad cow disease scare.

He saw his pro-business reform agenda put on hold as his government battled protests and political fighting in Parliament.

An officer at Seoul police HQ said investigators were looking for the source of leaflets that said the results of the probe on the navy ship sinking had been fabricated.

Some leaflets say the government has been taking pictures of its troops as preparations for their funerals in anticipation of war with North Korea, he said.

"We want to look at how these things are intended to influence the election," the officer, requesting his name be withheld, said.