Mitt Romney paid taxes on his multimillion-dollar income at a rate far below that of the average American, his tax returns reveal.

The returns, which the Republican presidential contender had steadfastly refused to release until now, show Romney earned $45m (£29m) over the past two years and paid just $6.2m in taxes, a rate of about 15%. Most of the income was derived from investments from his $200m-plus fortune.

The documents show Romney's income places him in the top 1% of earners in the US, which is only likely to add to the political embarrassment around the release of the returns because of its resonance in the age of the Occupy movement.

Romney's rivals for the Republican nomination will tread carefully in attempting to exploit his vast income and low taxes, in part because they have it as an article of faith that government spending, and therefore taxes, are far too high. But if he is the Republican presidential nominee against Barack Obama, the Democrats can be expected to portray his extreme wealth and relatively low taxes as a prime example of how the rich do not pay their fair share while shifting the burden to the less privileged.

It will also undermine Romney's attempts to make out he is an just an ordinary American who "lived in the real streets of America" and "didn't inherit money from my parents" when his critics portray him as a greedy corporate raider whose firm, Bain Capital, made large profits by throwing people out of work.

In Tuesday's Republican debate, shortly before his returns were made public, Romney offered up a defence.

"I pay all the taxes that are legally required and not a dollar more," he said. "I don't think you want someone as the candidate for president who pays more taxes than he owes."

But he recognised that it could be a political problem.

"You'll see how complicated taxes can be. And will there be discussion? Sure. Will it be an article? Yeah. But is it entirely legal and fair? Absolutely. I'm proud of the fact that I pay a lot of taxes," he said.

The returns show Romney paid a smaller proportion of his income in tax than most Americans because his income is from investments from his $200m-plus fortune, which attract a lower rate than earned income.

In 2010, Romney earned more than $21m and paid $3m in tax, a rate of less than 14%. He expects to pay $3.2m in taxes on his 2011 income, a rate of 15.4%.

He and his wife, Ann, donated $4.1m to the Mormon church and about $3m to other charitable causes. The money given to a church that many on the Republican right do not regard as Christian may not sit well with some voters.

Romney is still refusing to release tax returns for earlier years, leading some critics to speculate that he paid even less tax back then, before he knew he was going to run again for president.

Newt Gingrich revealed at the weekend that he paid nearly $1m in income tax, a rate of about 31%.

The Obamas had a joint income of $1.7m in 2010, much of it from sales of the president's two popular books. They paid 26% in tax.