The operation to repatriate hundreds of Senegalese illegal migrants back home from the Spanish Canary Islands has been suspended.
More than 700 illegal migrants reached the Canaries on Monday
Senegal will not co-operate further with Spain as the first group to return complained of mistreatment by the Spanish authorities, officials say.
Correspondents say this a setback to Spain's diplomatic efforts to stem the flood of people trying to reach Europe.
Some 7,500 migrants have arrived in the Canary Islands in the Atlantic in 2006.
About 1,000 more are believed to have died making the dangerous 1,000km crossing in small fishing boats.
Some of those sent back to Dakar on Wednesday said they had been handcuffed and falsely told they were on their way to Spain.
"We were handcuffed. Each of us was escorted by two policemen," Samba Ndao told reporters.
He said they were ordered to keep the plane's window blinds lowered during the flight.
"To our huge surprise, we landed in Dakar," Mr Ndao said.
Unnamed diplomats said it was hard to persuade those who had made the hazardous journey to the Canaries to go home.
"If you tell these people that they are going to be flown back home, then they won't get on the plane," Reuters news agency quoted one Dakar-based diplomat as saying.
"We'd hoped the repatriation would be carried out in a dignified manner. That wasn't the case," a senior Senegalese official told Reuters.
A Spanish foreign ministry spokesman confirmed to Efe news agency that the operation had been stopped "temporarily" until the Senegalese allegations had been investigated.
Last month, Spain announced a three-year diplomatic drive in West Africa to try to halt the flow of African migrants to Europe.
Would-be migrants have turned to the sea crossing from West Africa to the Canary Islands as a result of increased surveillance and tighter security on routes through Morocco - both across the Mediterranean and to Spain's enclaves in North Africa.