Republican presidential front-runner Donald Trump pledged to "tax Wall Street" as he sought to use asevere stock market selloff to plant new seeds of fear among voters during a campaign rally on Saturday in Ottumwa Iowa.

At times the real estate mogul sounded more like Democratic presidential candidate Senator Bernie Sanders a constant critic of Wall Street than a billionaire candidate running for the Republican presidential nomination.

"There's a bubble" Trump told his audience in southeastern Iowa noting the nation's high level of debt. "You see the stock market is starting to you know see what's going on" he said.

"It's starting to have some very bad weeks and some very bad numbers."

Volatility skyrocketed in financial markets last week as anxiety about global growth increased. The Standard & Poor's 500 Index tumbled six per cent on the week the biggest drop since September 2011 and the steepest slide over five days to begin a year on record.

Trump saidhis financial experience wasright for such troubled times."I'm really good at that stuff" he said.

"I know Wall Street. I know the people on Wall Street. We're going to have the greatest negotiators of the world but at the same time I'm not going to let Wall Street get away with murder. Wall Street has caused tremendous problems for us. We're going to tax Wall Street."

Trump also highlighted his independence from campaign contributions.

"I don't care about the Wall Street guys" he said. "I'm not taking any of their money."

The theater where Trump spoke seated about 700 and was filled an hour before his scheduled appearance forcing hundreds more to stand in an indoor space set up with a video feed.Appearing in the top US corn-producing state Trump took several shots at Senator Ted Cruz of Texas who is giving Trump a strong challenge especially in the campaign for the February 1 Iowa caucuses the nation's first nominating contest for 2016.

"My primary opponent is totally opposed to ethanol and the ethanol industry because he's with the oil industry" Trump said.

Joining with the ethanol lobby Trump has criticised Cruz for opposing the federal Renewable Fuel Standard which sets the minimum amount of corn-based ethanol and other biofuels that must be included in the nation's gasoline supply.

The junior senator from Texas has said that he supports ethanol but doesn't want to see the federal government pick "winners and losers" in the energy sector.

Trump also continued a line of attack against Cruz from earlier in the week suggesting that nominating him might be a dangerous thing for Republicans because Cruz was born in Canada.

"He's got to straighten out his problem" Trump said. "You can't have that problem and go and be a nominee."

Trump noted that the first-place finisher in recent Iowa caucuses had not ultimately won the party's nomination something he said he wants to change. In 2012 Iowa narrowly picked former Senator Rick Santorum of Pennsylvania and in 2008 former Arkansas Governor Mike Huckabee won by a wide margin.

"You haven't been good at picking the winners folks"Trump said. "If you pick me you're going to pick a winner." A Real Clear Politics average of recent state polls shows Trump running second in Iowa behind Cruz but the New Yorkerdid nothing to lower expectations for his performance in the state and said he would be spending "a lot" of time there before February 1.

"We've got to win Iowa" he said. "Otherwise we're wasting our time folks." Asked in an NBC News interview on Saturday about speculation he'd quit the race if he loses in Iowa Trump said "There's no 'maybes.' I'm not leaving."

Shawnda Starr 39 a pre-school teacher from Centerville said she changed her voter registration from independent to Republican last week so that she can caucus for Trump.

Asked if Trump had ever said anything that concerned her Starr said no including his proposed temporary ban on allowing Muslims into the US.