Mayoral candidate Ben Hall didn't hold back at a press conference on the steps of City Hall.
"It is with a heavy heart and quite regrettable that I have to announce this morning that the City of Houston is a victim of a mayor who is a shakedown artist."
Hall accuses Mayor Parker of running a pay-to-play system.
And he says city contractors are being threatened that they won't get any more work from the city if they support Hall's campaign.
Hall says he's asked the Harris County District Attorney to get involved and alluded to, but wouldn't confirm, the possibility of a grand jury investigation.
Meanwhile, Mayor Parker's campaign calls the allegations absurd.
Campaign Spokesperson Sue Davis says the contribution's to Parker's election campaign are legal and ethical and that Parker has nothing to hide.
"We'll have a serious discussion with Ben Hall about ethics as soon as he comes clean about not paying his back taxes and why he took a million dollar job from someone to whom he steered a city contract to."
Davis is referring to a contract given to John O'Quinn's law firm when Hall was the city attorney.
The O'Quinn firm later hired Hall away from the city when Hall was under investigation for allegedly steering contracts to certain companies.