Four times the tonnage of the original Open Arms rescue tugboat and with a capacity to carry up to 1,000 people, the Open Arms Uno made its first rescue on Wednesday, picking up 101 migrants stranded on a wooden boat off the Tunisian coast.

"Sit down, if you don't sit down we don't continue," a crew member shouted to the excited migrants from a speed boat launched from the vessel operated by the Spanish charity Open Arms before all were taken aboard.

One had jumped into the water to try to reach the speed boat, prompting it to briefly move away in a safety maneuver.

"People were showing signs of dehydration. They had been sailing for a day and were already adrift. We were able to find them quickly and they are now safely on board the Open Arms," lifeguard Mauro Di Si told Reuters.

The new Open Arms flagship has a 26-bed hospital, carries four semi-rigid speed boats and is designed to carry out mass rescue operations safely in challenging sea conditions.

Built in Norway in 2000, it was donated by Argentine-Italian filmmaker and philanthropist Enrique Pi?eyro, and is one of the largest maritime rescue vessels in Europe.

Nearly 25,000 people have lost their lives or disappeared since 2014 trying to cross the Mediterranean to Europe, including more than 1,100 so far this year, according to data from the International Organization for Migration (IOM).

The vessel is patrolling the deadliest migration route in the central Mediterranean.

The charity's original Open Arms tugboat, which is over 50 years old and has rescued more than 7,300 people at sea since 2017, has in the past few months been used to deliver food to the Odessa region in Ukraine and evacuate refugees amid the Russian invasion.

(Writing by Andrei Khalip; Editing by Hugh Lawson)