The New-York based organization said Moroccan police had brought in more than 100 people for questioning about the distribution of pro-boycott leaflets or other efforts to urge voters not to cast a ballot on Friday.
"The rate of voter participation will be closely watched because it is seen as a gauge of public enthusiasm for the reforms that King Mohammed VI initiated during 2011," Human Rights Watch said in an emailed statement.
"Some groups have urged a voter boycott, saying that the palace-led reforms do not go far enough to enhance the separation of powers and curb royal prerogatives.
"Harassing people who support a boycott is just as bad as harassing those who support a particular party or candidate, and casts a shadow over the vote," it added.
King Mohammed backed constitutional changes and brought the vote forward by 10 months as part of a plan by the palace to bring fresh faces into a government associated in the minds of many Moroccans with corruption.
But the pro-boycott camp, led by a group called the February 20 Movement, says the vote just promises more of the same.
The official MAP news agency on Monday denied that the police had arrested anyone for leading the boycott campaign after newspaper reports of several arrests linked to the boycott campaign.
"Summoning scores of boycott activists in cities around the country to police stations for questioning amounts to a state policy of harassment - whether or not they are formally arrested and eventually charged," Human Rights Watch said.
A law governing the Moroccan parliament reserves punishments of one month to one year in prison and a fine of 10,000 to 50,000 dirhams ($1,200 to $6,000) for "anyone who attempts, through the use of false information, false rumors, or any other fraudulent means, to change the vote of voters, or to push one or more voters to refrain from voting".
Human Rights Watch considered that law, which was implemented in October, to be incompatible with "strong affirmations of human rights, including freedom of expression" under the new constitution adopted in July 2011.
(Reporting By Souhail Karam)