The author said the £150 million development proposal for Edinburgh's Caltongate was "torpid, tactless" and would bring "shame and mortification" on those behind it.
Developer Artisan Real Estate Investors has insisted the plans, creating a civic square, hotels, retail units and affordable homes, will generate £43 million for the local economy a year and around 2500 jobs. Its proposals have been scaled back from those initially put forward by another firm.
The plans are due to go in front of Edinburgh City Council today but McWilliam voiced her opposition, singling out in particular a glass-fronted building planned for the site. She said: "No clarity of thought unites this proposed edifice ... the fundamentals of architecture, strength, beauty and purpose, are betrayed and with them countless citizens as yet unborn."
A spokesman for Artisan Real Estate Investors said it had carried out an 18-month consultation on the plans and had listened to the responses.
He added: "We are now delivering on our promises made when we started this process 18 month ago, bringing capital investment of £150 million to the table with the vision and commitment needed to complete what has already been started."