President Ali Abdullah Saleh, in power for 33 years, signed a deal granting him immunity from prosecution in return for stepping down. Last month, Saleh, a one-time U.S. ally against al-Qaida, transferred his powers to his vice president and said he intended to travel to the United States for medical treatment of wounds suffered in a June assassination attempt.
Many protesters have objected to the deal, which was brokered by Gulf Arab nations and supported by the United States, because it doesn't include far-reaching political changes or allow for Saleh to be tried.
The slogan for Friday's protests referred to the uprising's unmet aims: "Together to achieve the goals of the revolution."
Protesters carried pictures of Saleh with the words "You will not escape punishment."
"Saleh will not get immunity no matter what price we have to pay," said activist Mahmoud Taha.
The U.S. was still evaluating Saleh's request for a visa. Fearful of appearing to harbor an autocrat with blood on his hands, the Obama administration was trying to ensure that Saleh visits only for medical care and doesn't plan to stay, U.S. officials said this week.
Saleh was a key U.S. partner in battling al-Qaida in the Arabian Peninsula, which Washington believes is the most dangerous of several offshoots of the terror network.