Existing laws in the Gulf emirate which stipulate the protection of migrant workers were being largely overlooked, said James Lynch, Amnesty's researcher on foreign workers in the Gulf.
"The combination of forms of exploitation in certain cases that we have documented, we would consider that to amount to forced labour," he told AFP in an interview.
Lynch said the human rights watchdog's findings, based on weeks its staff spent in Qatar, would be detailed in a report to be published in November.
In 2010 oil-rich Qatar won the race to host the World Cup, football's showpiece event.
It is facing mounting pressure to improve conditions for migrants working on massive infrastructure projects, following claims of exploitation by World Cup organisers amounting to "modern-day slavery," as put in a report by British newspaper The Guardian last week.
"In the construction sector, unfortunately, exploitation appears from our research to be disturbingly normalised," said Lynch.
"In some cases, where certain kinds of exploitations are combined, that amounts to forced labour.
"The government has the responsibility to protect workers from abuses," he said, pointing out that Amnesty had presented its findings to the Qatari authorities and was expecting a response "imminently".
Lynch said forms of exploitation included the confiscation of passports, prevention of workers from leaving the country, withholding wages for long periods of time, and imposing financial penalties for absence.
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"Ninety percent of low-income workers report having their passports confiscated," he said, pointing out that the Amnesty team had spoken with workers from South Asian countries.
Lynch said the authorities should implement existing laws that set limits on working hours, and provide decent accommodation, health and safety measures, and monthly payments.
"There are some key protections laid out in the labour law, which were they to be implemented would clearly and significantly improve the labour rights in Qatar," said Lynch.
"Construction companies talk about these rules as being optional," and breaking them was seen as normal, he added.
On the other hand, the government should revise the labour law, mainly the sponsorship system that allows employers to prevent workers from changing jobs, or leaving the country, by denying them exit permits.
"We would want to see a really fundamental reform in the sponsorship system, and the exit-permit system is a key part of that," Lynch said.
Foreign nationals, regardless of their jobs, need an exit permit to leave Qatar at anytime.
Difficult conditions for migrant workers remain an issue across the oil-rich Gulf region, but some countries have introduced strict measures to improve their situation, including monitoring wages payments.
Last week's Guardian report said dozens of Nepalese have died working in Qatar in recent weeks, raising concerns about the country's preparations to host the World Cup.
Qatar has denied the report, and its labour ministry on Wednesday named a global law firm to "independently review" all the such claims made in the media.
The executive committee of FIFA, world football's governing body, is meeting on Thursday and Friday in Zurich to discuss the timetable for the tournament after calls for it to be staged in winter because of the desert emirate's scorching summer.