By Ingrid Melander and Marine Pennetier

PARIS (Reuters) - An angry President Francois Hollande on Saturday promised a "merciless" response to a wave of attacks by gunmen and bombers that killed 129 people across Paris, describing the assault claimed by Islamic State as an act of war against France.

As a cross-border investigation gathered pace, prosecutors said the coordinated assault on restaurants, a concert hall and the national football stadium appeared to have been carried out by a multinational team with links to the Middle East and Belgium as well as home-grown French roots.

In the worst carnage, gunmen systematically killed at least 89 people at a rock concert by an American band at the Bataclan music hall before blowing themselves up as anti-terrorist commandos launched an assault, officials said.

Some 40 more people were killed in five other attacks in the Paris region, including a double suicide bombing outside the Stade de France stadium, where Hollande and the German foreign minister were watching a football game.

The mass bloodshed came as France, a founder member of the U.S.-led coalition waging air strikes against Islamic State in Syria and Iraq, was on high alert for terrorist attacks, raising questions about how such a complex operation, which prosecutors said involved three separate teams, could go undetected.

It was the worst such attack in Europe since the Madrid train bombings of 2004, in which 191 died.

Hollande said the attacks were organised from abroad by Islamic State, with internal help. Investigators were focussing on to what extent the militants were from France or from abroad.

Three people were arrested in Belgium as part of an anti-terrorism probe centred on a Belgian hired car found near the site of one of the Paris attacks, Belgian prosecutors said.

Sources close to the inquiry said one of the dead gunmen was French with ties to Islamist militants and had been under surveillance by the security services.

The holder of a Syrian passport found near the body of one suicide bombers passed though the Greek island of Leros in October, a Greek minister said.

A Greek police source said the man had arrived in Leros with 69 refugees, where he was registered and had his fingerprints taken. Police declined to give his name.

If confirmed, the infiltration of a militant to carry out attacks in Europe into the flow of refugees could have far-reaching political consequences.

REFUGEE FALLOUT

The attacks fuelled a debate raging in Europe about how to handle the influx of hundreds of thousands of refugees and other migrants propelled by civil war in Syria, Iraq and Libya.

In a sign of potential divisions ahead, Poland said that the attacks meant it could not now take its share of migrants under a European Union relocation plan. Many of the migrants currently flooding into Europe are refugees from Syria.

The carnage on the streets of the French capital follow recent attacks claimed by Islamic State on a Russian passenger plane killing 224 people and bombings in Lebanon in which 43 died, all linked to the war in Syria.

Prime Minister Manuel Valls said France had no intention of halting its air strikes. Turkish President Tayyip Erdogan urged world leaders gathered for a summit in Turkey on Saturday to prioritise the fight against terrorism, saying the Paris attacks showed the time for words was now over.

Hollande cancelled his attendance at the G20 summit after declaring the first nationwide state of emergency since 1945. France will be represented by its foreign and finance ministers.

"Faced with war, the country must take appropriate action," the president said in a solemn address after an emergency meeting of security chiefs. He announced three days of national mourning.

Flags flew at half-mast, cinemas theatres and other places of entertainment were closed,, although schools and universities will reopen as normal on Monday.

"France will be merciless towards these barbarians from Daesh," Hollande said, using an Arab acronym for Islamic State.

Speaking after peace talks on Syria in Vienna, U.S. Secretary of State John Kerry said "we are witnessing a kind of medieval and modern fascism at the same time".

In its claim of responsibility, Islamic State said the attacks were a response to France's military campaign.

It also distributed an undated video in which a bearded militant warned in Arabic: "As long as you keep bombing you will not live in peace. You will even fear travelling to the market."

The Paris public prosecutor said 129 people were killed and 352 wounded, of whom 99 remain in a critical condition. Six attackers blew themselves up and one was shot by police. There may have been an eighth attacker, but this was not confirmed.

Relatives and friends scoured Paris hospitals on Saturday in search of people missing since Friday evening and believed to have gone to the Bataclan concert hall. Some anguished next of kin said their relatives were neither on the confirmed death toll nor among the wounded registered in hospitals.

STATE OF EMERGENCY

Hollande temporarily reimposed border controls as part of the state of emergency to stop perpetrators escaping or new attackers entering the country.

Local sports events in Paris were suspended, stores closed, the rock band U2 cancelled a concert, and schools, universities and municipal buildings stayed.

However, France said a global climate change summit in Paris at the end of the month would go ahead amid heightened security.

Emergency services were mobilised, police leave was cancelled, 1,500 army reinforcements were drafted into the Paris region and hospitals recalled staff to cope with casualties.

Sylvestre, a young man who was at the Stade de France when bombs went off there, said he was saved by his cellphone, which he was holding to his ear when a metal bolt hit it.

France has been on high alert since Islamist gunmen attacked the satirical weekly Charlie Hebdo and a kosher supermarket in January, killing 18 people.

Those attacks briefly united France in defence of freedom of speech, with a mass demonstration of more than a million people. But that unity has since broken down, with far-right populist Marine Le Pen gaining on both mainstream parties by blaming immigration and Islam for France's security problems.

World leaders responded to deadly attacks in Paris with defiant pledges of solidarity. From Barack Obama to Vladimir Putin and across Europe and the Middle East, leaders expressed their condolences

France ordered increased security at its sites abroad. Britain, Germany, Italy, Russia, Belgium, Hungary and the Netherlands tightened security measures.

British police said the evacuation of London's Gatwick Airport on Saturday was connected to the discovery of a possible firearm in a bin and a 41-year-old man from France was arrested.

POINT-BLANK

Julien Pearce, a journalist from Europe 1 radio, was inside the Bataclan when the shooting began.

In a television interview, Pearce said several young individuals armed with rifles entered the hall during a performance by Californian rock group Eagles of Death Metal and started "blindly shooting at the crowd".

"There were bodies everywhere," he said.

The gunmen shot their victims in the back, finishing some off at point-blank range before reloading and firing again, Pearce said, after escaping into the street by a stage door, carrying a wounded girl on his shoulder.

(Additional reporting by Geert de Clercq, Jean-Baptiste Vey, Emmanuel Jarry, Elizabeth Pineau, Julien Pretot and Bate Felix Tabi-Tabe; Writing by Giles Elgood; Editing by Sonya Hepinstall and Peter Millership)