Investment guru Warren Buffett has defended his proposal to tax the super rich at his annual general meeting in Omaha, Nebraska.

President Barack Obama has trumpeted the so-called "Buffett rule", which would introduce a flat-rate tax on those with the very highest incomes.

Buffett, the world's third richest man, said: "When we are asking for shared sacrifice from the American people, I would at least make sure that the people with these huge incomes get taxed at a rate that is commensurate with the rate they got taxed at not long ago."

Buffett said he had calculated that he paid the lowest tax rate in his office. "There are people like me with huge incomes … I don't have tax planning, I don't have Swiss bank accounts," he said. And yet, he added, his tax rate was around 17% while the average was somewhere around 30%.

Buffett said he was paying a rate similar to his cleaning lady. "My wife wants this explained. She does not have a cleaning lady – it's the cleaning lady at the office," he added.

Buffett said there were at least three years when he was paying the lowest tax rate in the Berkshire offices despite making anywhere from $25m to $65m.

The Buffett rule – which would impose a tax surcharge on incomes over $1m – has been condemned by Obama's critics, including Mitt Romney. "Someone calculated that the taxes he [Obama] would raise in his Buffett rule would pay for 11 hours of government," Romney said recently.

Earlier this year it was revealed that the presumptive White House challenger paid just 14% in tax on earnings of $21.6m in 2010.

One shareholder asked – to a round of applause – if Buffett should be making political statements, and said one investor he knew had been put off investing in Berkshire by Buffett'ss stance on taxes.

"I don't think any employee in Berkshire should have their citizenship restricted," Buffett said, in a nod to his own freedom to speak his mind.

He added of the dissenting voice in the audience: "It's fine if they disagree with us, [although] I think it's kind of silly. It sound to me like he ought to own Fox."

Buffett also used the meeting to reassure investors that cancer will not shorten his career.

About 35,000 people descended on Omaha this weekend to see the multi-billionaire investor deliver his annual address at the shareholder meeting for his investment firm, Berkshire Hathaway. Bill Gates and Bono were in the audience at the CenturyLink Center in Omaha. Among shareholders, Buffett's health was a major concern..

Last month Buffett announced that he had early-stage prostate cancer. He said he was told by his doctors that his condition "is not remotely life-threatening or even debilitating in any meaningful way".

"I feel real good," Buffett said on a walkabout before the meeting opened this morning.

The first question asked at the meeting was about succession. Earlier this year Buffett announced that an individual had been chosen to take over for him when he can no longer handle the chief executive duties, but he didn't say who that was.

Buffett joked that he wasn't "an arts major" and said that the candidate was capable of doing "many things better than I can." Asked if his successor could broker the same kinds of deals that have made Buffett so successful, Buffett said "not every deal that I've made" could be done by his successor. But the chosen individual would have his or her own talents.

Returning to his health, he told shareholders: "In all seriousness, it is a non-event," adding: "I feel terrific."