The demonstrators vow to remain at the hall until their demands are met
The 10th-seeded Goffin was 3-2 up and on game point when chair umpire Cedric Mourier ruled a Nadal shot was in but the Hawkeye system, which is not being used by officials on clay, showed the ball was long.
The point was replayed and fourth seed Nadal went on to break back before winning 10 of the remaining 11 games to set up a meeting with fellow Spaniard Albert Ramos in Sunday's final after the 15th seed beat France's Lucas Pouille 6-3 5-7 6-1.
The Monte Carlo Country Club crowd booed Mourier and a dejected Goffin did not shake the umpire's hand at the end of the match. "I imagine the ball was out but I was on the other side, I could not see the mark," said Nadal, who will play his 11th Monte Carlo final on Sunday.
Goffin made a brilliant start, playing aggressively and breaking in the third game as nine-times French Open champion Nadal looked clumsy.
The fourth-seeded Spaniard returned long on game point in the sixth game and the line umpire called the ball out but he was over-ruled by Mourier, who got off his chair to show Goffin a mark - the wrong one. "It's wrong, it's wrong," said Goffin, who eventually lost the 17-minute game after wasting other game points.
The Belgian never recovered and Nadal blazed through the rest of the match, which he won in less than an hour.
Earlier, Albert Ramos Vinolas beat Lucas Pouille 6-3, 5-7, 6-1 to reach the final. Although the 15th-seeded Ramos Vinolas has never previously reached a Masters final and has won only one career title, he has good credentials on clay, having got to the French Open quarterfinals last year.
The semifinal saw two players who made significant breakthroughs last year, with the 23-year-old Pouille reaching Grand Slam quarterfinals at Wimbledon and the US Open. Pouille had won their only previous match two years ago, in straight sets, on outdoor hard courts in Auckland.
Ramos Vinolas took the first set when he broke the 11th-seeded Frenchman to love, concluding with a smash at the net.
Pouille missed a chance to break in the ninth game of the second set, meaning he was serving to stay in the match. After holding easily with a neat volley at the net, Pouille got the crowd going and then played his best tennis of the match. He broke Ramos Vinolas in the next game when the Spaniard hit a forehand long, and Pouille levelled the match on his own serve.
But he seemed to be struggling physically in the deciding set.
At 3-0 down, Pouille needed treatment to his lower back and hips for about four minutes during the changeover.
His power went after that and the inevitable conclusion happened when Pouille dropped his serve again in the seventh game. Ramos Vinolas has one career title - on outdoor clay by the sea in Bastad, Sweden - and has lost three other finals, including this year on outdoor clay at Sao Paulo.
The demonstrators vow to remain at the hall until their demands are met
The volcano sent a tower of ash more than five kilometres into the sky as well as a fiery column of lava
It is too dangerous to make the 30-km journey from Jabalia to Rafah to see her son Yehia, says Sondos, mother of the baby
Carriers faced disruptions to flights after Iran's missile and drone attacks on Israel further narrowed options for planes
The guide covers everything from improving fine motor skills to teaching kids to care about the environment
This was the lowest demand in the past 9 months
At least 46 people were killed in a mudslide and flash floods in Mai Mahiu town in central Kenya
The airline conducts regular recruitment drives in cities across the globe to foster opportunities for aspiring aviation professionals