A Spanish woman has been served a one-year jail sentence after posting Twitter jokes about the 1973 assassination of a key ally to General Franco, as the number of convictions handed down for "justifying terrorism" by Spain's National Court continues to rise dramatically.

Luis Carrero Blanco was killed in a bomb attack by members of the Basque separatist group ETA, six months after being made prime minister.

He was seen as the heir apparent to the ageing dictator, and his death in an explosion which flung his car into the air opened the way for democratic reforms when Franco died two years later.

But four decades later, Agence France-Presse reports that 21-year-old Cassandra Vera has been landed with a criminal record after using Twitter to joke: "ETA combined a policy against the use of official vehicles with a space programme."

In another post, she asked: "Did Carrero Blanco also go back to the future with his car?"

The Spanish National Court ruled that the tweets "constitute contempt, dishonour, disrepute, mockery and affront to the people who have suffered the blow of terrorism", causing an outcry among human rights campaigners in Spain.

Spain election: Conservatives win, but without majority

The leader of left-wing populist party Podemos accompanied her to the high-profile trial, while the granddaughter of the assassinated prime minister herself said she was "scared of a society" where tweets led to jail sentences.

A Twitter search reveals hundreds of similar jokes posted before and after her arrest.

The number of court cases addressing such crimes has risen from 12 in 2015 to 37 in 2016, with 12 rulings already handed down in the first quarter of 2017.

In one high-profile case, singer-songwriter Cesar Strawberry was sentenced to a year in prison for joking about an ETA kidnapping on Twitter, while a group of puppeteers have also fallen foul of the courts' increasingly harsh application of the law.

The conservative People's Party has also introduced a new "gag law" allowing police to fine people for perceived contempt of authority.

Fines have been given for Facebook posts criticising local authority decisions, and one woman was charged for carrying a bag with the slogan 'ACAB – All Cats are Beautiful'. Police interpreted it as meaning 'All Cops Are Bastards'.