* Erdogan demands end to protests
* More than 90 demonstrations in cities across country
* U.S., EU voice concern over casualties
* Demonstrations reflect deeper tensions over Erdogan's rule (Adds arrests, protests near PM's Istanbul office, EU reaction)
By Nick Tattersall and Humeyra Pamuk
ISTANBUL/ANKARA, June 1 (Reuters) - Turkish Prime Minister Tayyip Erdogan made a defiant call for an end to the fiercest anti-government demonstrations in years on Saturday as authorities arrested almost a thousand people in protests across the country.
Riot police backed by armoured vehicles and helicopters fired tear gas and water cannons in Istanbul and Ankara for a second day. Interior Minister Muammer Guler said 939 arrests had been made in more than 90 separate demonstrations.
The unrest was triggered by government plans for a replica Ottoman-era barracks housing shops or apartments in Istanbul's Taksim Square, long a venue for political protest, but has widened into a broader show of defiance against Erdogan and his Islamist-rooted Justice and Development Party (AKP).
Crowds of protesters chanting "shoulder to shoulder against fascism" and "government resign" marched on Taksim, where hundreds were injured in clashes the day before. Broken glass, rocks and an overturned car littered the square as night fell.
A helicopter buzzed overhead as groups of mostly young men and women, bandanas or surgical masks tied around their mouths, used Facebook and Twitter on mobile phones to try to organise and regroup in side streets. Police clashed with protesters who lit fires in the streets leading to Erdogan's Istanbul office.
"If this is about holding meetings, if this is a social movement, where they gather 20, I will get up and gather 200,000 people. Where they gather 100,000, I will bring together one million from my party," Erdogan said in a televised speech.
"Every four years we hold elections and this nation makes its choice," he said. "Those who have a problem with government's policies can express their opinions within the framework of law and democracy."
Earlier, police pulled back from Gezi Park in Taksim, where the demonstration started peacefully on Monday with people pitching tents in protest at trees being torn up for the redevelopment.
Waiters scurried out of luxury hotels lining the square, on what should be a busy weekend for tourists in one of the world's most visited cities, ferrying lemons to protesters, who squirted the juice in their eyes to mitigate the effects of tear gas.
"People from different backgrounds are coming together. This has become a protest against the government, against Erdogan taking decisions like a king," said Oral Goktas, a 31-year old architect among a peaceful crowd walking towards Taksim.
Stone-throwing protesters also clashed with police in the Kizilay district of central Ankara as a helicopter fired tear gas into the crowds. Riot police with electric shock batons chased demonstrators into side streets and shops.
There were also protests in the Aegean coastal city Izmir.
Erdogan said the redevelopment of Gezi Park was being used as an excuse for the unrest and warned the main opposition Republican People's Party (CHP), which had been given permission to hold a rally in Istanbul, against stoking tensions.
But the protests included a broad spectrum of people opposed to Erdogan and were not organised by any political party.
CHP officials called on its members not to take party flags with them to the protests, apparently concerned they would be held responsible for the violence, and party leader Kemal Kilicdaroglu accused Erdogan of behaving like a dictator.
"Tens of thousands are saying no, they are opposing the dictator," he said. "The fact that you are the ruling party doesn't mean you can do whatever you want."
Erdogan has overseen a transformation in Turkey during his decade in power, turning its once crisis-prone economy into the fastest-growing in Europe.
He remains by far the country's most popular politician, but critics point to what they see as his authoritarianism and religiously conservative meddling in private life in the secular republic, accusing him of behaving like a modern-day sultan.
Tighter restrictions on alcohol sales and warnings against public displays of affection in recent weeks have also led to protests. Concern that government policy is allowing Turkey to be dragged into the conflict in neighbouring Syria by the West has also sparked peaceful demonstrations.
Medics said around 1,000 people had been injured in the clashes in Istanbul. At least four lost their eyesight after being hit by gas canisters, while four more were being treated for fractured skulls, the Turkish Doctors' Association said.
The U.S. State Department said it was concerned by the number of injuries while European Union foreign policy chief Catherine Ashton called for restraint on all sides.
Erdogan acknowledged mistakes had been made in the use of tear gas and said the government was investigating, but said the police reserved the right to use reasonable force and vowed that the redevelopment plans for Taksim would go ahead.
"Taksim Square can't be a place where extremist groups hang around," Erdogan said of a location which has long been a venue for mass demonstrations. (Additional reporting by Evrim Ergin, Can Sezer and Murad Sezer in Istanbul, Parisa Hafezi in Ankara, Justyna Pawlak in Brussels; Writing by Nick Tattersall; Editing by Jon Hemming)