ISTANBUL (AFP) - Turkish Prime Minister Recep Tayyip Erdogan was due back in Turkey on Thursday to face thousands of angry demonstrators calling for him to quit, as the nationwide unrest claimed a third life with the death of a policeman.

Away for four days on an official trip to North Africa, Mr Erdogan has defied the protesters who are against the conservative policies of his Islamic-rooted Justice and Development Party (AKP), dismissing them as "extremists" and saying everything would calm down before he returned.

But the mass unrest has only intensified in his absence, with doctors reporting thousands of injured as the police tried to quell the rallies in major cities with tear gas, pepper spray and water cannon.

A Turkish policeman died in hospital on Wednesday hours after being injured in a fall while chasing anti-government protesters in the southern city of Adana, the private NTV news channel said.

His death marks the first police fatality in seven days of clashes, with two young male protesters killed earlier.

Fresh violence broke out in the capital Ankara overnight, but Istanbul, where the unrest started on May 31, was relatively calm after six nights of unrest.

Protesters in Istanbul's Gezi Park next to Taksim Square, the epicentre of the protests, said they were bracing for Mr Erdogan's return from Tunisia on Thursday evening.

"I'm afraid because I don't know what he will do," said chemistry student Ezgi Ozbilgin, 24, who camped out in the park overnight.

"AKP supporters are like sheep. If Erdogan says go fight, they will fight. If he says stay, they will stay."

Mr Erdogan earlier said that he could "mobilise a million supporters of my party" if he had to.

Thousands of striking union members joined the anti-government protests on Wednesday, marching in Istanbul and Ankara in a sea of red and white union flags, drumming and yelling for Mr Erdogan to resign.

Opposition to him is intense, but the 59-year-old has won three elections in a row and gained almost 50 per cent of votes in 2011, having presided over strong economic growth in recent years.

The pro-AKP half of Turkey has remained largely silent in the past week of unrest, but CNN-Turk television reported on Thursday that several hundred people attacked a group of 25 youths who staged an anti-government protest in the Black Sea port of Rize, Mr Erdogan's home town.

In a hint that the wave of outrage could become two-sided, Deputy Prime Minister Huseyin Celik has urged AKP supporters not to flock to the airport to welcome Mr Erdogan back to avoid inflaming tensions.

"The Prime Minister does not need a show of power," he told a local television channel.

A heavy-handed response to a peaceful demonstration in Istanbul last week sparked nationwide protests denouncing Mr Erdogan, who has been in power at the head of the AKP since 2002.

His critics say he has become increasingly authoritarian. They have accused him of seeking to force conservative Islamic values on Turkey, a mainly Muslim but staunchly secular nation.

The national doctors' union has said more than 4,300 have been injured in recent days, 47 of them seriously, while at least 10 had lost their eyes in the unrest.

Turkey, while acknowledging some police excesses, has hit back at criticism of its handling of the crisis, a Turkish Foreign Ministry source told Agence France-Presse on Wednesday.

The diplomat, speaking on condition of anonymity, said Foreign Minister Ahmet Davutoglu had told United States Secretary of State John Kerry in a phone call Tuesday: "Turkey is not a second-class democracy."

The US State Department later denied suggestions this was Washington's view of its Nato ally, a key strategic partner in the region, but said it had concerns "about instances of police brutality".

Other Western allies of Turkey - a country that straddles East and West and has long aspired to join the European Union - have also voiced concern over the violence.

Mr Davutoglu reportedly assured Mr Kerry an investigation was under way into the police response.

In the Premier's absence, Deputy Prime Minister Bulent Arinc has apologised to wounded protesters.

On Wednesday, he met protest representatives who urged him to sack the police chiefs in cities where officers had used excessive force and to ban the use of tear gas against protesters.

But his gestures did not calm the protests, which have thrown up the biggest challenge to Mr Erdogan's decade-long leadership.

"He (Erdogan) doesn't understand what's going on here," said 30-year-old Ms Ozlem Altiok as she sat in the shade of a tree in Gezi Park, the small patch of green whose preservation fight sparked the flame of the unrest. "We won't be quiet until he goes."

Mr Erdogan has told protesters they should wait to express their views in elections next year, when observers expect him to run for president.