"I have appealed the decision by the prosecutor," Claes Borgström told AFP. "I have asked a higher-ranked special department in the prosecution's office of Sweden to reconsider the [prosecutor's] decision."
A Swedish duty prosecutor on Friday night issued an arrest warrant for Assange over an allegation of rape, but chief prosecutor Eva Finn abruptly withdrew it on Saturday, saying new information had come to light.
On Wednesday, she said there was no reason to believe a crime had been committed in that case, but said she had enough evidence to continue probing a separate molestation allegation against the 39-year-old Australian.
Borgström said that he and Finné had reviewed the same material and that he "obviously" had a different opinion to the prosecutor.
"She has not spoken to the two women as I have, they have told me the same thing they told the police...and I come to a different conclusion than the prosecutor," Borgström said.
Assange said on Wednesday that he would collaborate with Swedish police. He has repeatedly denied the allegations, saying the claims are part of a "smear campaign" aimed at discrediting his whistleblowing website, which is locked in a row with the Pentagon over the release of secret US documents about the war in Afghanistan.
WikiLeaks published nearly 77,000 classified US military documents on the war in Afghanistan on July 23rd and has said it will publish another 15,000 within the next couple of weeks.