Six children and their mothers were among at least 28 people killed on a bus by gunmen claiming to be members of a Honduran revolutionary group opposed to the death penalty.
Police arrested a suspect who was driving a car similar to that identified by witnesses as having been present at the attack in Chamelecon, 125 miles north of the capital, Tegucigalpa.
He is an alleged member of violent gangs that have terrorised residents, mostly in the poor neighbourhoods of the Central American nation's major towns.
The suspect was carrying a .38-calibre pistol and several automatic weapons, said police spokesman, Deputy Commissioner Wilmer Torres.
"It was an unbelievable massacre," he said.
The gunmen left a large piece of paper taped to the bus windscreen with a message saying they represented a revolutionary group that opposes the death penalty.
The message contained "vulgar words" against President Ricardo Maduro, congressional President Porfirio Lobo Sosa and Security Minister Oscar Alvarez, Torres said.
The note warned that "people should take advantage of this Christmas, because the next one will be worse," Mr Torres said.
The attack came just two days after Alvarez announced that authorities had uncovered plans by drug traffickers and local criminals to assassinate Maduro and his family, as well as himself.
The bus was driving through the heavily populated neighbourhood of San Isidro when a car cut in front of it, forcing it to stop, Mr Torres said.
The assailants jumped out of the car and began shooting, at the same time that other attackers in a second car fired from behind and then alongside the bus, he said.
The police spokesman said more than 50 people were aboard the bus, the majority women and children.
Many were coming home with bags filled with gifts and food for Christmas, Mr Torres said.
Sixteen of the victims were killed on the bus while 12 others, including two children and 10 women, died after being taken to hospital.
The bus driver was among the dead.