The former prime minister launched an attack on the SNP's tax policies under independence as he joined Alistair Darling at a Better Together rally aimed at postal voters.
Speaking at the event in Dundee, Mr Brown told the audience that while the nationalists "dine out" on ideas of equality, they have "no plans to raise funds that would come from a fairer taxation system".
"They dine out on Scottish ideas of equality - they talk as if they actually believe it," he said.
"But when you look at the actual policies of the SNP, there is not one measure in their document that suggests there would be a higher rate of income tax for those at the very top, or a millionaire's tax at the top of council tax, or a mansion tax at the top of stamp duty, or even the bankers' bonus tax that is proposed for the UK.
"They have no way of raising the money to pay for all the expensive promises they have made."
Mr Brown said SNP proposals to cut corporation tax would benefit large companies, including energy firms.
"The biggest beneficiaries of the SNP's tax policy are the shareholders and directors of the privatised energy companies in Scotland," he said.
"The beneficiaries of an independent Scotland are not the ordinary people of Scotland but the richest directors of the most profitable, privatised companies in Scotland.
"When you look at the Scottish National Party policies, inequality and poverty will survive until doomsday if Alex Salmond is all that confronts it."
The former prime minister delivered his rallying call for the union despite being heckled by a member of the audience.
The man, who was reported to have given a false name to gain entry to the event at Dundee's Caird Hall, shouted "rubbish" and "you're an absolute disgrace" before he was removed from the meeting midway through Mr Brown's address.
The rally, which came as people who have chosen to vote by post in the September 18 referendum receive their ballots, saw Mr Brown and Mr Darling share the stage despite well-documented disagreements in the past.
Mr Darling, leader of the pro-UK Better Together campaign, served as chancellor when Mr Brown was in charge at 10 Downing Street.
Mr Brown said: "It is a pleasure to sit alongside Alistair Darling, who for the last two years has done the incredibly difficult job, which we must now thank him for, of bringing together all the parties under the banner of Better Together."
Earlier, the rally heard from football commentator Archie Macpherson, who said he had accepted an invitation to speak out of "anger".
"Sitting watching the obfuscation and evasion streaming out of the independence campaign. I couldn't stand it any longer," he said.
Mr Macpherson looked back to the end of the Second World War in 1945, when he said victory had transformed British society into "something we must never forget".
"Our forebearers created the welfare state, the education system for which myself and others have hugely benefited," he said.
"I intend to stick on that particular road, difficult though it may be.
"Within the United Kingdom that is the right and moral road, because I am as concerned about food banks in Liverpool as I am in Glasgow or Dundee."
Commenting on the Yes campaign's argument that the future of the NHS is threatened by a No vote, he added: "My wife and myself have benefited enormously from the NHS and it turns my stomach to see the SNP trying to make political capital out of this extraordinary piece of work.
"The television debates are accentuating the feeling that this is, one, a personality contest, and two, that it's a general election in which we are voting for a president. It is nothing of the kind.
"I am voting for my kids and their kids... it hasn't sunk into people. It is your job to go out and question these people who are thinking about voting Yes.
"Ask them to provide the evidence, ask them to provide certainty. There is no certainty. It is a gamble, as Alistair said."
He added: "It is a huge gamble and I am not prepared to take it.
"I value the things we have at the moment because we were together, and we should stay together.
"The wrong road is the yellow brick road which they (Yes supporters) are riding... which ends in deception, deceit and fantasy."
Commenting on the rally, former Scottish Labour chairman and Labour for Independence member Bob Thomson said: "This is a sign of the panic spreading in the ranks of the No campaign, following Monday's debate drubbing and the fact that more and more Labour supporters are moving to Yes - at the last count already more than 230,000.
"But just as this photocall can't paper over the cracks in the relationship between Mr Darling and Brown, it won't paper over the cracks in a No campaign which lost the debate, is losing the argument and is losing voters all over Scotland who are moving towards a Yes vote on September 18."