The former head of South Korea's main spy agency accused of meddling in last year's presidential election denied all charges against him through his attorney in a Seoul court on Monday.

Won Sei-hoon served as the National Intelligence Service (NIS) chief under former President Lee Myung-bak until early this year.

He was indicted on charges of ordering an online smear campaign against opposition candidates in an aim to sway public opinion in favor of the ruling party prior to the Dec. 19 election.

The defendant "denies all charges filed against him," Won's lawyer said during a preliminary hearing at the Seoul Central District Court in southern Seoul.

Won was specifically charged with violating both the election law and the law governing the status of NIS officials who are banned from intervening in domestic politics.

Prosecutors say the former chief instructed his subordinates to write more than 1,000 political articles and replies on the Internet from September till early December, including those praising then government and ruling party candidate Park Geun-hye and slandering opposition candidate Moon Jae-in.

Won, who did not attend the preparatory hearing as it was not a requirement, will make his first court appearance sometime in mid-August, according to court officials.

In a separate case, prosecutors said Friday they were seeking an arrest warrant for Won on charges of accepting bribes from a construction company in return for peddling influence.

Hwang Bo-yeon, head of now-bankrupt Hwangbo Construction, had testified that he had been giving the former NIS chief some 100 million won (US$87,565) in cash and gifts since 2009 in exchange for his influence in securing major construction projects by public institutions and large corporations.

After 11 hours of overnight questioning by prosecutors, Won was allowed to go home early Friday. Investigators said he had denied taking bribes.

A hearing to decide whether to arrest Won will be held at the Seoul court on Wednesday, according to court officials. (Yonhap News)