The scandal had saddled President George Weah with his biggest political crisis since he took office in January. He has yet to comment in detail on the case and his cabinet ministers have given differing accounts of what happened to the money.
"This money is for our country, for our children, for tomorrow," said Precious Williams, 43, who braved a rainstorm and a heavy police presence to protest in the seaside city. "We are here to get our money back. We want justice for our missing money."
The funds amount to about 5 percent of the annual GDP of Liberia, whose donor-reliant economy has been ravaged by two civil wars, the second of which ended in 2003, and an Ebola outbreak from 2013 to 2016 that killed thousands.
The government said last week that the Liberian dollars, newly printed abroad, had been unaccounted for since arriving in containers at two ports in November 2017 and August of this year. That triggered an investigation and angry reactions from across the West African country.
In an address on state radio on Friday evening, Weah urged the public to remain calm, said any wrongdoing committed in connection with the money would be punished and reiterated campaign promises to fight corruption.
That did little to quell anger towards the president.
On Monrovia's streets on Monday, protesters chanted: "George Weah, act now. We didn't elect you to steal."
Weah's predecessor, Nobel Peace Prize-winning Ellen Johnson Sirleaf, has also come under fire. Her son, Charles Sirleaf, and 14 other Liberians have been banned from leaving the country while the investigation is underway. He has not been reachable for comment.
The U.S. government is considering a request from Liberian officials to aid the investigation, and central bank governor Nathaniel Patray said in a statement on Monday that the bank was cooperating with investigators.
Meanwhile, frustration mounts.
"We cannot go to other countries lobbying for money then 16 billion Liberian dollars just goes missing," said 21-year-old political activist James Zeah. "I want them to bring back our money immediately."