The online retail giant orders hundreds of agency staff to turn up before dawn – but pays them only if there is work to do, an employment rights centre has reported.
Earlier this week around 200 workers who were transported to the centre at 6am were told to wait, unpaid, in a canteen for two hours before starting work, the Inverclyde Advice & Employment Rights Centre said.
Some of the workers are foreign students, but many are unemployed people forced to take agency work by the Job Centre.
Jim McCourt, of the IAERC, said: "As usual at this time of year Amazon employs a large number of workers, employed by agencies such as Blue Arrow, CDR and RTO, to name a few, to help cope with demand. In recent week our offices have had reports of workers being booked off shift due to the lack of work.
"On one occasion, workers were told to sit in the canteen, without pay, because a lorry was delayed.
"They are told these posts are for a specific number of hours. They are often worse off for working. Amazon are preying on the most venerable in our community, who are simply not in a position to protest."
IAERC has been critical of Amazon in the past. Last year the centre revealed that shifts of foreign workers were been cut short in the middle of the night.
Amazon has previously come under fire for its anti-union policies: a former Gourock employee is currently taking it to a tribunal claiming he was victimised because of his union activities.
But Amazon's record on industrial relations is now under scrutiny after it opened a new base in Fife, with 950 permanent jobs, thanks to nearly £9million in aid from various Scottish agencies.
First Minister Alex Salmond frequently highlights that employment success. Last month he described the new Fife base as "a testament to their continued success as a world-leading online retailer".
Labour MSP Duncan McNeil said: "This company has received significant levels of public money and is held up as a shining example by the Scottish Government yet does not recognise trade unions and basic employment practices."
Amazon did not comment on the complaints. But a spokesman said: "Christmas 2011 has been our busiest on record and on December 5th alone, we took orders for more than three million items.
"It has been a very busy Christmas for everyone in the Gourock fulfilment centre, including the many hundreds of temporary associates who we employee leading up to Christmas."