"If you don't (hit targets) you end up in a disciplinary, so they're just having to work, work, work, they're not allowed to talk to people, it's difficult to take a toilet break"
Members of the GMB at the company's fulfilment centre in Coventry voted to walk out on Wednesday in protest at a pay rise the union said is worth 50p an hour.
Amazon has said it already offers "competitive pay, comprehensive benefits, and excellent opportunities for career growth, all while working in a safe and modern environment".
GMB senior organiser Stuart Richards said: "Today, Amazon workers in Coventry will make history.
"They've defied the odds to become the first ever Amazon workers in the UK to go on strike.
"They're taking on one of the world's biggest companies to fight for a decent standard of living.
"They should be rightly proud of themselves.
"After six months of ignoring all requests to listen to workers' concerns, GMB urges Amazon UK bosses to do the right thing and give workers a proper pay rise."
Amanda Gearing, also a GMB senior organiser, speaking from the picket line, said: "We're here today because Amazon workers have said they've had enough."
She said staff who worked through tough conditions during the pandemic are just "trying to get decent pay".
"They (the centres) are pressure-cooker environments they work in, with the targets they're expected to reach," she added.
"They (Amazon) just wear them out, get rid of them, replace them."
Ms Gearing added: "After all that, they have just offered a 50p pay increase in the biggest cost-of-living crisis that we've had in decades.
"I think, when the workers have got nothing to lose, you see them coming out fighting."
She said the striking workers are "not militant" and "need a job", adding: "Like I say, they've put up with the conditions in those centres for a long time."
"Coventry has been open four years, we've had issues since it was open. Rugeley (Staffordshire) before that - they're just horrendous places to work," she claimed.
She said the main issue for workers is what she said are target-led performance measures, set by an "algorithm".
"So, imagine turning up to work not knowing if you're going to make the grade.
"If you don't (hit targets) you end up in a disciplinary, so they're just having to work, work, work, they're not allowed to talk to people, it's difficult to take a toilet break," she claimed.
"In one of the other fulfilment centres, we were having people urinating in a bottle because they didn't want to take the time out and go to the toilet because it becomes idle time.
"We've got injuries, we've got more ambulances coming to (this) site, than any other warehouses across the country, they've just got an appalling record when it comes to health and safety.
"I think we've been lucky there's been no fatalities within those centres, up to now."
An Amazon spokesman said: "Amazon is a safe place to work.
"The vast majority of ambulance call-outs to our buildings are related to pre-existing conditions, not work-related incidents, and as a responsible employer we will always call an ambulance if someone requires medical attention. That's just sensible."
Regarding toilet breaks, the company said: "You can use the toilet whenever you like.
"You can always go to the toilet when you like, log off, have a drink, speak to your manager etc - that's not an issue."
Ms Gearing said she is "hoping Amazon will get round the table", while adding: "I don't anticipate they'll be giving me a call anytime soon."
The company spokesman said: "Our employees have the choice of whether or not to join a union. They always have.
"We've made great progress in recent years and months in important areas like pay and safety.
"The fact is, we already offer competitive pay, comprehensive benefits, and excellent opportunities for career growth, all while working in a safe and modern environment.
"The unions know this.”
It is understood that of about 2,000 staff at the Coventry site, 178 voted in favour of strike action.
The Amazon spokesman added: "A tiny proportion of our workforce are involved.
"In fact, according to the verified figures, only a fraction of one pc of our UK employees voted in the ballot - and that includes those who voted against industrial action.
"We appreciate the great work our teams do throughout the year and we're proud to offer competitive pay which starts at a minimum of between £10.50 and £11.45 per hour, depending on location.
"This represents a 29pc increase in the minimum hourly wage paid to Amazon employees since 2018.
"Employees are also offered comprehensive benefits that are worth thousands more - including private medical insurance, life assurance, subsidised meals and an employee discount, to name a few."