The law took effect on Tuesday, which made the Central American nation the first country to adopt bitcoin as a legal currency.
"The government does not listen to the needs of the people, and the new law not only fails to benefit the country, but also hampers the economy of El Salvador," protesters said, as quoted by the Salvador news outlet.
Those protesting, including social and civil organizations and peasants, claim they do not want to use the cryptocurrency and see the move as a money laundering attempt.
The opposition accuses Salvadoran President Nayib Bukele of pursuing this policy for personal enrichment. They also note that bitcoin is legalized in light of possible dollar deficit.
On Tuesday evening, the bitcoin prices fell down by 9% after Bukele announced his country had purchased another 200 bitcoins. Later, El Salvador bought another 150 bitcoins.