Attorney Mario Joseph says he has been subjected to an escalating series of menacing phone calls, harassment and intimidation over the past year.
An adviser to Martelly on Friday called the allegations "completely false."
Joseph's work has involved several politically sensitive cases that include former dictator Jean-Claude Duvalier, who returned to Haiti last year following 25 years in exile and is facing corruption charges.
The alleged threats against Joseph and two other lawyers have drawn concern from Amnesty International, the Center for Constitutional Rights and the National Lawyers Guild. The three advocacy groups issued separate statements this week denouncing the harassment.