More than 2,000 head teachers from across England, Wales and Northern Ireland have marched on Downing Street protesting funding cuts and demanding ministers do not take them for "fools".
Around 1,000 heads were expected, but organisers of the march calling for increased funding for schools, but organisers say more than 2,000 took part.
The march was organised by grassroots campaign "Worth Less?" which believes extra investment will alleviate concerns about class sizes, staff recruitment and retention and teaching conditions.
The protesters gathered in Parliament Square before converging on Downing Street, where a delegation delivered a letter to Chancellor Phillip Hammond protesting over what they claim are "unsustainable" funding cuts and demanding ministers do not take them for "fools".
Paul Whiteman, General Secretary of the National Association of Headteachers (NAHT), called the march an "organic experience of school-leader frustration and anger".
He continued: "It's a fantastic turnout and although it's supported by the unions, it's not arranged by them.
Rob Kelsall, one of the organisers, said the protest had taken place "because there is no alternative".
"We are seeing schools - both maintained and academies - that are seeing their funds depleted, dipping into their reserves, and having to send out begging letters to parents.
”One teacher said to me that if we go to London on Friday and get the Treasury to change its funding policy, he reckons that will be the best day's work he's ever done. That's the stage we're at.“
Shadow education secretary Angela Rayner voiced support for the protest.
She said the "unprecedented action by headteachers is a clear sign of the desperate struggle they now face to provide a decent education while balancing the books."
Earlier this year, figures showed the number of secondary schools in England running at a loss had nearly trebled in four years.
The study, published by the Education Policy Institute in March, said the number of local council-run secondary schools in deficit dropped from 14.3% in 2010/11 to 8.8% in 2013/14, but between 2013/14 and 2016/17, the numbers in deficit nearly trebled to 26.1%.
In July, the Institute for Fiscal Studies said total school spending per pupil fell around 8% in real terms in England between 2009/10 and 2017/18.
In response to the march, a Department for Education spokesperson said: “There is more money going into schools than ever before, rising to a record £43.5 billion by 2020 - 50% more in real terms per pupil than in 2000.
”Every school attracts more funding per pupil through the National Funding Formula, high needs funding has risen to over £6 billion this year, and the 3.5% pay rise we announced for classroom teachers on the main pay range is backed by £508 million Government funding.
“We know that we are asking schools to do more, which is why we are helping them to reduce the £10 billion spent each year on non-staffing costs, providing Government-backed deals for things like printers and energy suppliers that are helping to save millions of pounds.”