The union head, Mouyyad al-Lami, said he hoped to visit his colleague later.

An Iraqi official said Mr Zaidi had been handed over to the judicial authorities, according to the AFP news agency.

Earlier, Dargham al-Zaidi told the BBC's Caroline Wyatt in Baghdad he believed his brother had been taken to a US military hospital in the Iraqi capital.

A second day of rallies in support of Mr Zaidi have been held across Iraq, calling for his release.

Meanwhile, offers to buy the shoes are being made around the Arab world, reports say.

Hero figure

Mr Zaidi told our correspondent that despite offers from many lawyers his brother has not been given access to a legal representative since being arrested by forces under the command of Mowaffaq al-Rubaie, Iraq's national security adviser.

Iraqi lawyers have speculated that he could face charges of insulting a foreign leader and the Iraqi Prime Minister, Nouri Maliki, who was standing next to President Bush during the incident. The offence carries a maximum penalty of two years in jail.

Our correspondent says that the previously little-known journalist from the private Cairo-based al-Baghdadia TV has become a hero to many, not just in Iraq but across the Arab world, for what many saw as a fitting send-off for a deeply unpopular US president.

As he flung the shoes, Mr Zaidi shouted: "This is a goodbye kiss from the Iraqi people, dog."

Dargham al-Zaidi told the BBC that his brother deliberately bought Iraqi-made shoes, which were dark brown with laces. They were bought from a shop on al-Khyam street, a well-known shopping street in central Baghdad.

However, not everyone in Iraq has been supportive of the journalist's action.

Speaking earlier in Baghdad, Mouyyad al-Lami described Mr Zaidi's action as "strange and unprofessional", but urged Mr Maliki to show compassion.

"Even if he has made a mistake, the government and the judiciary are broad-minded and we hope they consider his release because he has a family and he is still young," he told the Associated Press news agency.

"We hope this case ends before going to court."

Abducted by insurgents

The shoes themselves are said to have attracted bids from around the Arab world.

According to unconfirmed newspaper reports, the former coach of the Iraqi national football team, Adnan Hamad, has offered $100,000 (£65,000) for the shoes, while a Saudi citizen has apparently offered $10m (£6.5m).

The charity called on the media to support Mr Zaidi and put pressure on the Iraqi government to free him.

Mr Zaidi, who lives in Baghdad, has worked for al-Baghdadia for three years.

Muzhir al-Khafaji, programming director for the channel, described him as a "proud Arab and an open-minded man".

He said that Mr Zaidi was a graduate of communications from Baghdad University.

"He has no ties with the former regime. His family was arrested under Saddam's regime," he said.

Mr Zaidi has previously been abducted by insurgents and held twice for questioning by US forces in Iraq.

In November 2007 he was kidnapped by a gang on his way to work in central Baghdad and released three days later without a ransom.

He said at the time that the kidnappers had beaten him until he lost consciousness, and used his necktie to blindfold him.

Mr Zaidi never learned the identity of his kidnappers, who questioned him about his work before letting him go.