Edward and Sophie initially received a warm welcome to the island during a walkabout in the town of Soufriere, but were soon met by a group of around 10 protesters before a trip to a cocoa plantation.
One protester, who would not provide his name, told the PA news agency: “We want reparations now. The Queen of England needs to apologise for slavery.”
Another smiled and said: “London Bridge is falling down.”
The demonstrators displayed banners reading “repatriation with reparations” and “Queen say sorry” while playing drums and chanting.
The protests follow similar demonstrations in St Vincent and the Grenadines, where some held signs saying “compensation now” and “Britain your debt is outstanding”.
The earl and countess were also urged to use their “diplomatic influence” to provide “reparatory justice” by the prime minister of Antigua and Barbuda during a visit to the country on Monday.
Edward was criticised for his “disinterest” in Gaston Browne’s speech by anti-monarchist campaigners, after he was seen nervously laughing when asked to respond.
The earl joked that he had not been taking notes during Mr Browne’s opening remarks, so could not respond to all the points he had made.
The prime minister did not laugh at the comment.
Sophie and Edward initially experienced the warmest welcome they had received on their Caribbean tour during a walkabout in the town of Soufriere, with one man saying to Edward: “I salute you, my prince.”
The earl spoke to a road sweeper with a broom who was standing in front of the small crowd, and joked “I hope you keep this lot in order” while pretending to wave the broom.
Sophie and Edward were cheered as they left the area.
The couple arrived at the Main Jetty in Soufriere by boat on Wednesday ahead of a busy schedule of engagements.
The countess was presented with a bouquet of flowers by a boy named Carlos Baptise, 11, who has a spinal injury called congenital scoliosis.
The couple then travelled past the small group of protesters to the Fond Doux Cocoa Plantation where they learned about the contribution of cocoa to Saint Lucia’s economy.
The countess had a go at grinding cocoa beans before meeting representatives from female-led businesses to hear about the impact of Covid-19 on trade and tourism.
She also tried painting a purse at one of the stations, while the earl was taken to where the cocoa beans are dried in the sun.
The pair then visited Sulphur Springs, where they were told about the history and evolution of the volcano.
The countess put her hand in the water and said: “It’s as warm as a hot bath.”
Sophie and Edward then had lunch with Saint Lucia’s prime minister before travelling to Soufriere’s Mini Stadium, where the earl and countess watched a performance celebrating the island’s young people.
The couple joined in with dancers as they both waved small Saint Lucian flags.
They then met and posed for photos with youngsters from a number of schools, including Soufriere’s infant, primary, comprehensive and special education schools.