Chanting war songs, they said they wanted to present to Ellen Johnson-Sirleaf their complaints about the delayed severance payments, but she had already left the state radio premises.
"We wanted to meet the president. We do not think this is the right way for this government to treat us. We worked for this country for many years and up to now, nothing has been done for us," ex-Lieutenant Amos Kollie, one of the protesters, said.
Another protester, Jacob Toe, a former master sergeant, said: "We have families that we need to take care of." Jordanian peacekeepers from the United Nations Military Mission in Liberia and Liberian police fired tear gas canisters to break up the demonstration.
Two UN armoured personnel carriers patrolled the streets.
No injuries were reported but witnesses saw at least one police officer being beaten with a stick by protesters, whom police said were also armed with knives.
Since Johnson-Sirleaf took office as Africa's first elected female head of state early last year, she has faced several protests from former members of the old armed forces of Liberia.
Thousands of ex-combatants have been demobilised and paid off to make way for a new, slimmed-down Liberian armed forces, the first unit of which graduated in November after training by a US defence contractor, DynCorp International.