Many of some 50,000 demonstrators waved Croatian flags, photographs of Gotovina and banners insulting the authorities and chief UN war crimes prosecutor Carla Del Ponte.
"Ante = Croatia," "General, we are with you!" "We were defended by a hero, not a criminal!" read a few of the slogans.
Some demonstrators wore black T-shirts with the caption "Ante Gotovina -- pride of his homeland."
"For us Ante Gotovina is more than a general. He is a symbol of the Croatian people and his trial is a trial of the Croatian people," Zeljko Strize, head of a local veterans' association which organized the rally, told the crowd gathered on the waterfront of the Adriatic town of Split.
"From this gathering I want to send him a message: Ante we are all by your side."
The Mayor of Split, Zvonimir Puljic, and district chief Ante Sanader were whistled down when they tried to address the crowd.
Veterans are "ashamed of such authorities and their politics of toadying to world powers," Strize said. His words were welcomed by chants of "Treason! Treason!"
"What good is the state when we do not have the general who created it?" a 63-year-old pensioner from Split said.
Gotovina was arrested in Spain on Wednesday after having been on the run since mid-2001 when the UN's International Criminal Tribunal for the former Yugoslavia (ICTY) charged him over atrocities against ethnic Serbs in 1995.
The 50-year-old was detained in an ICTY cell at The Hague on Saturday pending the start of his trial.
His lawyer said he would plead not guilty before the UN court on Monday.
"At his first appearance general Gotovina will plead not guilty," lawyer
Luka Misetic told the HINA news agency, adding that he had spoked to his client after he had been placed in UN custody.
He said Gotovina was "relaxed and self-confident" and believed he would succesfully defend himself.
Gotovina is the most high-profile Croatian at The Hague and his arrest has been a cause of diplomatic tension for years.
The August 1995 operation over which he is accused practically ended Croatia's 1991-95 war of independence from the former Yugoslavia, and he is seen as a national hero by many Croats.
The veterans demanded his trial be transferred from the Netherlands to Croatia and that the government fight for his provisional release from UN custody as well as aid his legal defence.
No violence was reported at the protest after a series of incidents including roadblocks and burning of tires -- notably around the central coastal town of Zadar where Gotovina was born -- since his arrest.
Protest rallies were also held Sunday in the capital Zagreb, where some 500 people gathered at the main Ban Josip Jelacic square, as well as in several other cities.
In the Austraian capital Vienna another 300 nationalist protesters shouted slogans of Croatia's pro-Nazi World War II regime and called for the murder of Croatian President Stipe Mesic.
In neighbouring Bosnia some 150 people protested in the southern town of Mostar, which has a significant ethnic Croat population.
Thousands of people gathered at pro-Gotovina rallies in Zadar and the southern Adriatic town of Dubrovnik on Saturday. At least one protest on Friday included extreme nationalists displaying pro-Nazi paraphenalia.
The European Union and the United States had pressured Croatia to track down Gotovina since his indictment but the government insisted the former French legionnaire had fled abroad.
The EU, which Croatia seeks to join, made Gotovina's arrest a key condition for stronger ties.