The Excluded Unity Alliance said its first lobbying event in London aimed to highlight the “untenable financial situation” facing people who have missed out on meaningful support.
The Treasury said it has created one of the largest and most comprehensive Covid-19 support packages in the world.
However, campaigners estimate that three million people have fallen between the cracks and received no meaningful support during the pandemic.
The group met outside Parliament to hear from a range of speakers.
Torrin Wilkins, founder and director of the Centre Think Tank, said: "We have given this Government plan after plan on how to solve this.
"If the Conservatives want a message today, it's quite simple. You need to do the right thing. Not just for those people who are suffering but also because of the economy”.
The Conservative MP Esther McVey told the lobbying event: "People cannot live on fresh air, people cannot go on not knowing what is happening to them."
Ms McVey said the prolonged lack of financial support had caused an emotional and financial drain on the people affected.
Labour MP Paula Barker said she would continue to support the campaigners until they gained justice.
A Treasury spokesperson said: “Throughout this crisis our priority has been to protect lives and livelihoods.
“Our schemes have supported millions of jobs and the incomes of millions more. While we acknowledge it has not been possible to support everyone in the way they might want our schemes were designed to target support at those who need it most, while protecting public money against error, fraud and abuse.”
The spokesman added: "The Coronavirus Job Retention Scheme (CJRS) has helped to pay the wages of people in 11.4 million jobs across the country and the Self-Employment Income Support Scheme (SEISS) has received claims from 2.7 million self-employed workers.
·"More people have become eligible for employment support schemes. The Government has moved the cut-off date on which employees need to be with their employer to be eligible for the Coronavirus Job Retention Scheme. For claims starting on 1 May 2021, the cut-off date has been moved from 30 October 2020 to 2 March 2021.
"This means that those employed on 2 March 2021 are now eligible for CJRS if the other eligibility criteria are met. The Self-Employment Income Support Scheme grant has been extended and is now based on 2019-20 tax returns. This has brought more self-employed people into the scope of the scheme, including many new to self-employment in 2019-20.
"Throughout this crisis, we have done all we can to support jobs and livelihoods through our £350 billion package of support, and our self-employed and furlough schemes are among the most generous in the world."