* Kidnapping of Shi'ite pilgrims triggered Shi'ite protests
* Abductions followed Sunni battles in the Lebanese capital
* U.N. says security forces, rebels, violate human rights (Recasts with SNC official)
By Mariam Karouny and Khaled Yacoub Oweis
BEIRUT/AMMAN, May 25 (Reuters) - Lebanon said on Friday that a group of Lebanese Shi'ites kidnapped in Syria had been freed and were safe in Turkey, but produced no sign of the hostages at the centre of a kidnap drama heightening tensions over the conflict in neighbouring Syria.
The twist in a hostage drama that has inflamed political tensions in a country divided between foes and friends of the uprising in Syria came as Syrian activists said government troops killed at least 50 people in the centre of the country.
The account of the killings underscored the relentlessness of the violence in Syria's 14-month-old revolt against President Bashar al-Assad, which has thrown Lebanon's delicate sect-based politics into the country's worst unrest in years.
Lebanese Interior Minister Marwan Charbel told reporters at Beirut airport the hostages were in Turkey, had been delayed for "logistical reasons" and would reach Lebanon in hours. He said they were in good health, and speculated they were being questioned by Turkish officials.
He was explaining to a restive crowd why the captives had failed to appear hours after Prime Minister Najib Mikati's office said that Turkey had confirmed release of the hostages, who were snatched by gunmen in northern Syria as they returned from a religious pilgrimage this week.
A member of the fragmented body that claims to speak for Syria's political opposition raised new doubts over their fate, saying their captors still held them.
"We are still working on the handover. They are still with the armed group," Ahmad Ramadan of the opposition Syrian National Council (SNC) told Reuters, speaking from Istanbul.
HEZBOLLAH RENEWS CALL FOR CALM
The kidnappings followed the worst unrest in years in the Lebanese capital, where rival Sunni factions loyal and opposed to Syria fought street battles after the killing of a Sunni Muslim cleric opposed to Assad in northern Lebanon.
Fighting had erupted in that region two weeks ago between Sunni Islamist gunmen, the Lebanese army and Lebanese members of the Alawite sect to which Syria's rulers belong over the arrest a Lebanese Islamist suspected of funnelling arms to the Sunni Muslim insurgents fighting against Assad.
Residents of Beirut's southern suburbs - a stronghold of Hezbollah, the Syrian and Iranian-allied guerrilla group and political movement - rushed into the streets to celebrate news the captives would be freed, with women ululating and tossing rice in the air.
Those same areas saw enraged Shi'ites burn tyres and block the road to Beirut's airport as word of the kidnappings spread.
Hezbollah leader Hassan Nasrallah, who had appealed for calm after the kidnappings, said on Friday they would not sway its support for Syria.
Near midnight in Beirut, Hezbollah and another pro-Syrian Shi'ite Muslim faction issued a statement calling on those who gathered at the airport to meet the captives to go home until further notice and keep calm.
ACTIVISTS: SYRIAN FORCES SHELL TOWN
As Lebanon awaited the hostages, Syrian activists said government troops shelled Houla in Homs province, killing at least 50 people including 13 children after shooting an anti-Assad protester.
"The soldiers are shelling Houla right now, the casualties are huge," activist Ahmad Kassem said. He said opposition fighters fired back, inflicting casualties on the soldiers and destroying five tanks.
The Syrian state news agency - which gives the government's view of the uprising as a foreign-funded conspiracy - said six people were killed by a bomb planted by "terrorists" in the central city of Homs late on Thursday.
Syrian security forces earlier killed at least four anti-government protesters on Friday when they opened fire on demonstrations in the northern city of Aleppo and the outskirts of the capital Damascus, activists said.
The fringes of the capital also saw skirmishes between rebels of the Free Syria Army and government forces who fired on demonstrations in the Zamalka district, activists said.
There was no independent confirmation of any of those accounts from within Syria, which has limited access for journalists during the uprising.
The United Nations, which six weeks ago put forth a plan for a ceasefire aimed at paving the way for a political end to Syria's bloodshed, said on Friday that recent bomb attacks in Syria may have been the work of "established terrorist groups."
"The sophistication and size of the bombs point to a high level of expertise, which may indicate the involvement of established terrorist groups," U.N. Secretary-General Ban Ki-moon told the Security Council in a letter on Friday.
A day earlier, the global body said both Syrian government forces and the insurgency that has sprung up alongside what began as a mass protest movement had committed serious human rights abuses.
In a report documenting 207 killings since March, U.N. investigators said government forces had executed entire families in their homes and rebels tortured and killed captive soldiers and government supporters.
The global body said it can no longer track casualties in the uprising, after estimating at least 9,000 people have been killed. Syria says "terrorists" have killed about 2,600 security and military personnel.
The envoy overseeing the U.N. peace plan, Kofi Annan, is due in Syria shortly, Annan's spokesman said. It would be his first visit since presenting the plan, which includes teams monitoring the ceasefire, which has yet to take hold. (Writing by Joseph Logan; Editing by Michael Roddy and Xavier Briand)