* Sudden drop in migrants leaving Libya
* New group is preventing them from leaving
* Group made up of civilians, policemen, army figures
By Aidan Lewis and Steve Scherer
TUNIS/ROME, Aug 21 (Reuters) - An armed group is stopping migrant boats from setting off across the Mediterranean from a city west of Tripoli that has been a springboard for people smugglers, causing a sudden drop in departures over the past month, sources in the area said.
The revelation throws new light on the sharp reduction in migrant arrivals from Italy, which took over from the Aegean route as the main focus of European concerns in the crisis.
Arrivals in Italy from North Africa, the main route for migration to Europe this year, dropped by more than 50 percent in July from a year earlier, and August arrivals so far are down even further. July and August are peak months for migrant boats because of favourable sea conditions.
Sources in Sabratha, 70 km (45 miles) west of the capital, said the sudden drop had been caused by a new force in the seaside city, which is preventing migrants from leaving, often by locking them up.
The group in Sabratha "works on the ground, the beach, to prevent the migrants leaving on boats towards Italy," said a civil society organiser from the city, speaking on condition of anonymity.
The group is made up of several hundred "civilians, policemen, army figures," he said. It is conducting a "very strong campaign" that was launched by a "former mafia boss", said a second Sabratha source who follows smuggling activity closely.
A third source with contacts in Libya, who also asked not to be named, said the Sabratha group was making "a significant effort to police the area".
The two Sabratha sources said the group was running a detention centre for migrants who are turned back or taken from smugglers. One sent a picture of hundreds of migrants sitting in the sand in front of a high wall.
One of the sources said he thought the group was seeking legitimacy and financial support from Tripoli, where European states have tried to partner with a U.N.-backed Government of National Accord (GNA) to stem migrant flows. An official from the interior ministry's department for combating illegal migration in Sabratha did not respond to a request for comment.
It was not possible to contact the group, which the third source said was called Brigade 48, although other sources did not confirm this.
Italy has been trying to bolster the GNA's ability to stop people smuggling with cash, training and by sending a ship to help repair Tripoli's coastguard and navy vessels. Some 600,000 migrants have reached Italy by sea from North Africa since 2014, testing the country's ability to cope. More than 12,000 have died trying.
Most leave from Libya's western coast. Following a local backlash against smugglers in Zuwara in the west in 2015, Sabratha became the most frequently used departure point.
Italy wants to replicate a deal with Libya that the EU struck with Turkey last year, largely shutting down the migrant route through Greece and the Balkans.
With a national election looming during the first half of next year, the government in Rome is under pressure to show it can stop, or at least slow, migration.
But any progress in Libya is likely to be fragile, with the country in a state of conflict since Muammar Gaddafi was ousted six years ago. Rival governments are vying for power and local militias battle each other for territory and smuggling profits.
Last week Italy seized on the drop in arrivals, with Interior Minister Marco Minniti saying he saw a "light at the end of the tunnel".
LEAVING POINT SHIFTS
Migrants rescued last week in the Mediterranean confirmed that conditions had changed in Sabratha, according to a spokesman at the International Organization for Migration, which interviewed migrants who arrived in Trapani, Sicily, on Saturday.
"They said that it was very difficult to depart from Sabratha. There are people stopping the boats before they set out, and if they get out to sea they're immediately sent back," said Flavio Di Giacomo, an IOM spokesman in Rome. Some migrants were also turned back before reaching Sabratha, he said.
The European Union's border control agency Frontex last week said "clashes in Sabratha" contributed to July's decline, also citing changeable weather and increased Libyan coastguard presence. The Sabratha sources were not aware of any clashes.
Another shift in recent weeks has been a clampdown on smuggling of Bangladeshi and North African migrants through Tripoli's Mitiga airport, after a militia that controlled the trade was forced out by a GNA-aligned armed group at the start of July, Libyan and European officials said.
But that, like a slowing of flows into Libya through Niger, might take time to take effect. Hundreds of thousands of migrants are already in Libya.
In Sabratha, the changes may not stick.
In the past, with no central authority to constrain them, smugglers have adapted and routes have shifted, as already is happening.
Last week smugglers moved departures to east of Tripoli, near Al Khoms, Chris Catrambone, co-founder of the Migrant Offshore Aid Station (MOAS) charity, told Reuters. Three large rubber boats set out from the east, he said, while only a small boat with 26 people was found west of Tripoli.
"The sea was like a lake last week and yet there were few boats," Catrambone said.
Everyone on the Phoenix, a rescue vessel operated by MOAS, was taken aback because it was so unusual, he said.
The GNA has little control over armed groups in western Libya, including the capital, and none over factions that control the east of the country.
The civil society member from Sabratha said the new group there might stop working if it does not receive support from Tripoli.
The power of the smuggling networks would not be broken until there was a "legitimate source of order" in Libya, said a senior diplomat, speaking of the change in Tripoli airport and comparing the situation to broken vase.
"In one corner we stuck it together, but everything else is in pieces."
(Additional reporting by Ahmed Elumami; editing by Giles Elgood)
Our Standards: The Thomson Reuters Trust Principles.