Thousands of parents in England plan to keep their children off school for a day next week in protest at tough new national tests, campaigners say.
Parents from the Let Our Kids Be Kids campaign said children as young as six were labelling themselves failures.
In a letter to Education Secretary Nicky Morgan, they said primary pupils were being asked to learn concepts that may be beyond their capability.
The government said the tests should not cause pupils stress.
These new tests, known as Sats, have been drawn up to assess children's grasp of the recently introduced primary school national curriculum, which is widely considered to be harder than the previous one.
The letter from the campaign, which says it represents parents of six and seven-year-olds across the country, says children are crying about going to school.
It also says pupils' mental health is at risk because of the increased pressure they face.
"There are tens of thousands of us, and we have reached the point when it is time for us to speak," the letter to Mrs Morgan says. "We need you to listen."
But it adds: "We're not convinced, based on your track record, that you'll listen to just words so, to make it very clear how strongly we feel, we are also planning a day of action with a Kids' Strike on 3 May, which will see thousands of primary school children staying off school... in protest at the Department for Education's testing policies.
"We want an end to Sats now."
The campaign's website says: "In May, children in Year 2 sit a whole weeks' worth of exams.
"These children are six or seven years old.
"All year, their curriculum has been centred around comprehension and arithmetic in order to pass these tests.
"Outdoor learning has decreased, childhood anxiety has increased, games have been replaced with grammar, playing with punctuation."
Tuesday, May 3, was chosen specifically because it is a non-testing day.
The letter comes a week after the government cancelled its spelling and grammar tests for seven-year-olds, after the BBC News website reported this year's test paper had been accidentally published on a government website.
Let Our Kids Be Kids, being led anonymously to avoid the focus being put on certain localities or schools, says it expects thousands of children to be kept off school as part of the strike.
More than 10,000 people have posted details on Facebook of events they are organising on the day of the boycott, it says.
These include picnics in parks, woodland trails in forests and arts activities in places of local interest.
And a petition on the campaign's website calling for the tests to be scrapped has been signed by 26,000 people.
A DfE spokesman said: "We know mastering the basics of literacy and numeracy at primary school has a huge impact on how well children do at GCSE, which is why we are determined to raise standards.
"We have updated the Key Stage 2 tests to reflect our new, more rigorous curriculum, which will help every child fulfil their potential regardless of their circumstances.
"Tests help teachers identify and provide the support pupils need as well as giving parents a picture of how their child is doing."
Parents can face fines for taking their children out of school without permission.
The campaign has prepared a template letter for those wishing to protest to send to head teachers.