Former Caracas mayor Antonio Ledezma, who escaped a Venezuelan prison and fled to Spain in 2017, attended the rally in opposition to Venezuelan President Nicolas Maduro.
The gathering coincided with a major rally Guaido held in Venezuela, where tens of thousands of supporters in Caracas demanded that Maduro step down from power.
Loyalists of Maduro's socialist government flooded the streets in a different part Caracas to celebrate 20 years since Hugo Chavez launched the Bolivarian revolution.
There was another smaller rally for Guaido in Barcelona, Spain's second largest city.
The number of people born in Venezuela who live in Spain has jumped from 165,000 in 2015 to 255,000 last year, according to Spain's National Institute of Statistics.
Venezuela opposition leader Juan Guaido says he hopes Pope Francis will soon weigh in on the crisis.
Italian daily La Stampa asked Guaido in an interview in Caracas what the Vatican or the pope could do to avoid a worsening of Venezuela's political crisis.
The newspaper on Saturday quoted Guaido as replying: "We hope to soon hear a pronouncement by the pope."
But the Argentine-born pontiff said a few days earlier that although he is terrified there could be a bloodbath in Venezuela, his pastoral role doesn't let him take sides.
Guaido has declared himself interim president as he tries to force President Nicolas Maduro from power.
Venezuela President Nicolas Maduro has proposed holding early elections for the National Assembly as he seeks to outmaneuver a renewed opposition that is trying to use its control of the legislature to challenge his rule.
Maduro in a speech to supporters Saturday said it would be up to the pro-government constitutional assembly to decide whether or not to back his proposal.
Elections for the opposition-controlled National Assembly aren't supposed to take place again until 2020.
National Assembly President Juan Guaido is leading a charge to force Maduro from power after he took the oath of office last month for a second, six-year term widely considered illegitimate.
Venezuela opposition leader Juan Guaido says humanitarian assistance will begin flowing into the crisis-wracked country within the next few days, despite the objections of President Nicolas Maduro.
Guaido made the announcement Saturday speaking before tens of thousands of cheering supporters who took to the streets of Caracas demanding that Maduro step down from power.
Guaido said supplies of badly needed food and medicine will start entering from the Colombian border town of Cucuta. He said assistance will also come from two more unnamed entry points.
Loyalists of Maduro's socialist government flooded the streets in a different part of the capital Saturday to celebrate 20 years since Hugo Chavez launched the Bolivarian revolution.
Thousands of public employees and government supporters have begun to concentrate on Caracas' downtown Bolivar Avenue.
Some danced to popular songs, while a banner included a large photo of late President Hugo Chavez and read: "20 years of popular victories."
The socialist government of President Nicolas Maduro called the rally to celebrate the 20th anniversary of Chavez's rise to power.
Meanwhile, thousands of anti-government protesters are carrying Venezuelan flags and descending on the eastern part of the city.
Opposition leader Juan Guaido has proclaimed himself interim president of Venezuela and called for demonstrations to increase pressure on Maduro to resign.
Hundreds of Venezuelans gathered in downtown Barcelona, Spain, on Saturday to express their support for Juan Guaido, who declared himself interim president of Venezuela last week.
The gathering was one of several expected around the world to coincide with a rally planned by Guaido in Venezuela.
The defiant socialist government of President Nicolas Maduro has called on its own loyalists to flood the streets waving flags to celebrate the 20th anniversary of the Bolivarian revolution launched by the late Hugo Chavez.
Nancy San Juan, a 78-year-old yoga teacher, says: "This is the end. It's imminent. How is it going to end? We don't know. We don't know because this man (Maduro) doesn't want to leave and is very defiant."
Spain's National Institute of Statistics says the number of people born in Venezuela who live in Spain jumped from 165,000 in 2015 to 255,000 last year.
A Venezuelan air force general defected from the administration of President Nicolas Maduro on Saturday and called on his compatriots to participate in protests against the socialist leader's rule.
Gen. Francisco Yanez is the first high ranking officer to leave Maduro's government since Jan. 23, when National Assembly President Juan Guaido declared himself the country's legitimate leader.
In a YouTube video, Yanez said: "The transition to democracy is imminent." He described Maduro as a dictator and referred to Guaido as his president, but refused to say whether he is still in Venezuela or has left the country.
In a brief phone conversation with The Associated Press, the officer confirmed, from a Colombian number, the veracity of his declaration and said he would not give further statements until given authorization by "the commander-in-chief of the legal armed force which is President Juan Guaido."
In the video, Yanez claimed that "90 percent" of the country's armed forces are against Maduro.