The demonstrators came to the streets of the capital Madrid on Saturday, chanting "Spain, tomorrow, will be republican."
Meanwhile, left-wing political parties and civil groups have demanded a referendum on the issue.
Crown Prince Felipe is due to officially replace his father, 76-year-old King Juan Carlos, probably on June 19.
The event comes after thousands of people across the country took to the streets on June 02 following the king’s abdication.
Anti-monarchy activists formed the biggest rally in Madrid under the “Monarchy No Thanks” campaign while carrying Republican flags.
Thousands also gathered in Barcelona which seeks independence from central government as Catalonia’s capital. They chanted “Dear Philip, nobody has chosen you."
The Spanish King was long popular for the role he played in helping Spain's transition to democracy in the 1970s. However, he has lost public support in recent years over corruption scandals and blunders.
In 2012, Juan Carlos outraged the Spanish public as he took a secret luxury elephant-hunting trip to Botswana at the time one in four Spanish workers was jobless and the government was teetering on the brink of default.
A poll published by the Spanish newspaper El Mundo in January showed that fewer than half of Spaniards, or 41 percent, support the monarchy in general and nearly two-thirds of them want Carlos to abdicate.