The Honk For Hope protest has seen around 500 coaches from across the country make their way to Parliament Square in convoy.
Met Police issued a warning to the demonstrators after complaints about the noise were made from the Palace of Westminster.
Many social media users spotted the line of buses making its way into the capital along the A4, with the traffic said to reach back several miles.
Coach businesses have said they feel let down by the response of the Government during the coronavirus pandemic, with some claiming they face bankruptcy.
At a time when companies would normally be fully booked with summer tours, many are sitting empty, with staff still on furlough.
With the demand for coach trips falling, many drivers are also struggling to make finance repayments on their vehicles, which can be as much as £450,000.
The protesters are asking the Government for a package of support measures, including financial help to meet these payments and an extension of the furlough scheme for the industry.
Drivers are also asking for the removal of personal guarantees, which leave them personally responsible for the repayments on their buses and could even see them lose their homes if they are unable to keep up with the bills.
The organiser of the protest, Jenna Rush, from North East Coach Travel, said: “We know that we will not move again until next year, and we cannot meet our finance payments for the rest of the year without some form of government support.
“I have got a coach now where I still owe £160,000 finance, but because of Covid the coach has depreciated to around £100,000.
“So if that (loan) company does not give me any help, and I can’t pay, they will repossess and take it away. But there will still be a shortfall of £60,000.
“Because I have a personal guarantee, they’ll then come to me and possibly take my home.
“Every operator is in the position, it’s not just the risk of losing the business, it’s the risk of losing everything.”
Darren Read, a driver at Bakers, added: “It is just devastating.
“I came back off furlough because we had a few school contracts, but we were only taking out one or two kids and it’s not enough to pay the bills.
“It’s just such an uncertain time.”