Dozens of reporters, cameramen, photographers, journalists' trade union leaders and lawyers' representatives rallied in the capital, Islamabad, on Wednesday.
Alluni was convicted on Monday.
"Taysir is a journalist not a terrorist," read one banner, as protesters wearing black armbands marched towards Parliament Square chanting slogans including: "Do not demonise Muslims".
Later they handed a memo of protest to the Spanish embassy in Islamabad.
"It's utterly shameful that a so-called civilised country has convicted a journalist who was performing his duty," said Fauzia Shahid, president of Pakistan's National Press Club.
Alluni, who is Syrian-born but a naturalised Spaniard, was jailed by a Spanish court at the end of Europe's biggest al-Qaida trial, which began in April.
He interviewed the group's leader, Osama bin Laden, in Afghanistan weeks after the 11 September 2001 attacks in the United States.
Accused of acting as a financial courier to the group while in Afghanistan, Alluni, who had faced a maximum nine-year term, said in testimony he was only doing his job as a journalist.
Also on Wednesday, Jordan's Islamic Action Front (IAF) party and the country's 14 professional unions denounced the Alluni conviction.
"We condemn this unjust verdict ... and demand the immediate release of Taysir Alluni from jail," the IAF said.
It urged "all those concerned by relations of mutual respect between the East and the West ... to raise their voices to condemn this unfair verdict".
Jordan's unions, most of which are dominated by Islamists and opposition groups, branded the verdict "unfair and political" and urged the Spanish government to take steps to overrule the sentence.
"We call on the Spanish government which took a brave position by pulling its troops from Iraq ... to take the practical measures to stop this political verdict against an innocent person," a statement said.
The head of the press association, Tareq al-Momeni, issued a separate statement deploring the conviction.
"This is an unjust and unreasonable verdict. It is an attempt to stifle voices and a violation against the freedom of the press. It is a dangerous precedent in the history of the press and journalists," Momeni said.