The national day of action comes as new figures from the Department for Education show the total number of education, health and care (EHC) plans has increased by 11 per cent since 2018.
The number of new EHC plans also rose by 16 per cent - but only 60 per cent of plans were issued within the 20-week time limit last year, a drop from 65 per cent in 2017.
The event, organised by SEND National Crisis, saw thousands of parents, disabled children and young people and special needs workers join marches around the country to highlight a multi-million pound funding gap which is affecting the delivery of SEND services.
The group said the funding gap - predicted to reach £1.6bn by 2021 - was a "national scandal" which meant disabled children were being denied the support they were entitled to.
"It is an intolerable situation that means access to rights, equality, inclusion and the prospect of a bright future are being wrongfully denied to thousands of disabled children.
"This is not just a national crisis; it is a national scandal," said Poppy Rose, the group's co-founder.
The lack of funding for SEND support was having a "detrimental" effect on the mental health, life chances and outcomes of these young people, she added.
Two events organised by the group in London saw campaigners deliver a petition with more than 13,000 signatures to Downing Street followed by a rally in Parliament Square.
Twenty-six other marches were held in locations across the UK including Bristol, Hull, Widnes and Manchester.
The group, which says the funding gap for high needs SEND was at least £287m last year, is calling for the government to introduce changes which would help reverse the cuts in provision.
"We can no longer remain silent when our children are suffering from want of adequate government funding.
"We are demanding a necessary change to the framework to ensure workable regulatory controls, and to ensure SEND funding is ringfenced to ensure delivery precisely where it is most needed," said Nadia Turki, co-founder of SEND National Crisis.
DfE research highlighted by the group shows children and young people with SEND in England are six times more likely to be excluded than pupils without any special needs.
They also drew attention to the steep rise in complaints about SEND to the Local Government and Social Care Ombudsman - 150 per cent since 2015 - of which nearly 90 per cent were upheld by the ombudsman, the highest of any category investigated.
The group is reporting recent details released by Ofsted showing half of all local area SEND services have failed inspections as well as data from the special educational needs and disability tribunal which found parental appeals have increased by 80 per cent since the SEND reforms were introduced.
The campaign has attracted support from organisations such as the National Deaf Children's Society, the Disabled Children's Partnership as well as the National Education Union (NEU).
Kevin Courtney, NEU joint general secretary, said its own investigation had revealed special needs provision in England had lost out on £1.2bn since 2015 due to shortfalls in funding increases from central government.
The union said funding given to local authorities had failed to keep up with a rapidly increasing demand for SEND provision. The number of children and young people with an EHC plan has risen by 33 per cent since 2015 - in contrast with a six per cent increase in the high needs funding block over the same period.
"This is clearly a crisis, with pupils and parents bearing the brunt of real-terms funding cuts and the wholly inadequate planning by government.
"Last year, when the NEU won an additional £350m for children and young people with additional needs, the government admitted that ‘more needs to be done'.
"We hold them to those words today," said Courtney.