There are so many African migrants in Libya wanting to make the dangerous trip to Europe that Tripoli zoo has been turned into a processing centre for them.

With the country's 22 refugee centres overflowing, the zoo – closed since the 2011 uprising – is being used to handle those picked up on the streets. More than 50 people are brought there every day.

"The numbers arriving here are changing in an unbelievable way," said Said Ben Suleiman, deputy commander of 20 Support Brigade, which operates the detention centre. "We deport 10 and we find hundreds coming back."

The smuggling gangs stay one step ahead of the authorities. As police have stepped up their ability to catch people traffickers, so smugglers have developed a new and dangerous way to avoid capture.

"Traffickers make sure they're nowhere near the migrants when the boats arrive to pick them up," Suleiman said. "What they do, they buy a cheap boat, then they give one of the migrants the keys, so the captain is chosen from among them. It means they go to sea with no training."

He thinks this practice may have led to the Lampedusa boat disaster, as migrants head to the sea with neither training nor navigational skills.

However, he does not expect the disaster to deter would-be migrants. "These guys don't care, all they are thinking is get to Europe."

Many arrive in Tripoli unable to afford the journey, with prices for a place on a boat starting at €1,000 (£850). "They all have in the back of their mind that Europe is better than Libya, but the trip is a lot of money," he said. "They all want it [Europe], but they can't all afford it."

For those facing deportation it is hard to know what to feel: pity that all those hard miles have come to nothing, or relief they won't be attempting the parlous journey north.

Abdul Rahman Ali, a 27-year-old from Niger who is expecting deportation after being picked up by police, paid smugglers a total of 700 dinars (£350) to make the long journey north across the Sahara to Tripoli.

Dressed in a gold-and-white striped shirt and sandals, he said the first leg of his trip was the crossing from Niger over Libya's Sahara desert border to Sabha, capital of Libya's southern Fezzan province: "It was a four-day trip to Sabha in a 4x4. When we got there there was a new group [of smugglers] and we had to pay again.

"They took us in another pickup, 14 of us crammed under canvas so we would not be spotted. We drove up the main highway, but there are checkpoints so we kept having to detour through desert."

Ali said he made the journey with two friends: one became sick in Sabha and returned to Niger, the other was arrested two days after arriving in Tripoli.

Back home, he says, conditions are grim and wages low. "In Niger there is work, but its maybe 10 [dinar] a day. In Libya you can earn five times that. Everyone I know wants to migrate."

People smuggling is big business in a country where economic stagnation is the rule. The most popular embarkation point in the capital is not some out-of-the-way port, but Gagaresh, the capital's equivalent of Oxford Street.

A glass-fronted branch of Debenhams has just opened here, across the road from Marks and Spencer, but a few streets away the beachfront is home to a very different trade.

The long smooth beach is ideal for the people-smugglers to bring their boats, and local residents are used to the tell-tale lanterns of the migrants on the beach at night.

"Just before Ramadan we caught 90 people trying to leave Gagaresh beach in a little boat for Italy. They have pregnant women on these boats – there was a baby a few months old," said Suleiman.

The International Office of Migration said Libya has yet to request an assistance programme, and repatriation flights it runs for some Africans to return home are rejected by asylum seekers from the horn of Africa. "We have an active program of voluntary repatriation but nobody goes back to Somalia or Eritrea," said IOM spokesman Chris Lom.