Image copyright Reuters Image caption Zimmerman at the time of his arrest and Martin pictured three years before his death

An online auction for the pistol used to kill unarmed black teenager Trayvon Martin has apparently reached $65m (£45m), organisers say.

The sale has been plagued by fake bidders including "Racist McShootface".

George Zimmerman, who shot and killed the teenager, had planned to auction what he called "an American icon" on the website Gun Broker on Thursday.

But the web posting was removed just as the auction was due to begin with a reserve price of $5,000 (£3,450).

United Gun Group is now hosting the auction.

In a statement on Twitter they defended the sale of the gun on their site. They were "truly sorry" for the Martin family's loss but said it was their goal to "defend liberty".

"Unless the law has been violated, it is the intention of the United Gun Group to allow its members to use any of the available features. While not always popular this is where we stand."

Image copyright Getty Images Image caption Mr Zimmerman said the gun was recently returned to him

On Friday afternoon, the top bidder was a user named Craig Bryant.

Mr Zimmerman, 32, a neighbourhood watchman, was cleared over the death of the 17-year-old in February 2012 after saying he acted in self-defence.

Does the gun belong in a museum?

In an online posting to announce the auction, Mr Zimmerman said that he would use the profits to "fight" the Black Lives Matter movement and oppose Democrat Hillary Clinton's presidential campaign.

A lawyer for the Martin family told the Washington Post that "it is insulting to this family that he would decide that he would sell the gun that he killed their child with".

"Think about what that means: This is a gun that took a child's life and now he wants to make money off of it."

On the auction site, Mr Zimmerman said it was recently returned to him by the US Department of Justice.

He claimed that the Smithsonian museums had expressed interest in buying the 9 mm handgun, but Smithsonian officials denied that in a statement.

Speaking to a Florida television station, Mr Zimmerman had defended the auction saying "I'm a free American, and I can do what I would like with my possessions."

Image copyright AP Image caption Zimmerman has had several encounters with police since being acquitted

Analysis - Nick Bryant, BBC North America correspondent

Few cases in recent years have been more racially sensitive or led to such an anguished national conversation as the killing of Trayvon Martin. It sparked demonstrations around the country, prompted President Obama to remark that if he had a son, he'd have looked like the black teenager and brought about the first use on social media of the hashtag "Black Lives Matter."

So the decision of the former neighbourhood watchman, George Zimmerman to put the gun he used up for auction not only seems extraordinary but also cruel and callous - especially since he refers to the weapon on the online site as an "American icon."

This is not the first time that Zimmerman has sought to cash in on his notoriety. His first painting of an American flag, emblazoned with the words "God One Nation with Liberty and Justice For All," sold on eBay for the staggering sum of $100,000. But it did not impress critics, who called it "primitive" and "appalling."

Harsher language will no doubt be used to describe the sale of the pistol that killed Trayvon Martin.

Image copyright Getty Images Image caption Protests were launched nationwide following Martin's death, which helped to create the Black Lives Matter movement

Congressman Hakeem Jeffries said on Thursday that "Trayvon Martin's cold-blooded killer should be in prison. Instead, he is trying to profit from the stunning miscarriage of justice."

Florida police did not arrest Mr Zimmerman for six weeks after the shooting in Sanford, Florida, provoking mass rallies in Florida and throughout the US.

Police justified their decision not to detain him by citing the state's controversial "stand your ground" law, which allows a citizen to use lethal force if he or she feels in imminent danger. Police initially said the law prevented them from bringing charges.

Profiles: Trayvon Martin and George Zimmerman

Mr Zimmerman's defence said Trayvon Martin had punched their client, slammed his head into the pavement and reached for Mr Zimmerman's gun. Prosecutors accused Mr Zimmerman of telling a number of lies.

The case led to protests in several cities in the US and to the birth of the Black Lives Matter movement.

Mr Zimmerman's name has been in news headlines several times since his closely watched trial.

Twice, assault charges involving his girlfriend were dropped.