George Weah, a former international football star, told hundreds of supporters on Sunday that he would work to block the inauguration in January of Ellen Johnson-Sirleaf, who won last month's election to become Africa's first elected female head of state.
Weah rejects the results, claiming fraud.
"There is no victor for now, and I say there will be no inauguration in the country until the world gets together and finds a means for a peaceful resolution to the problem," said Weah, back in Liberia after meeting regional leaders.
Echoing the wartime bombast of Liberia's numerous warlords and factional leaders, he said: "Fellow partisans, revolution is a noble cause. We must fight to obtain it.
"It is our right to seek justice, and we will use all means to obtain that. I know one day we will be free."
Riot police spread out in streets near the site of Weah's address, and small, scattered skirmishes erupted between the security forces and Weah supporters.
Johnson-Sirleaf is Africa's first
female elected head of state
The elections for a leader to take over from a transitional administration arranged under peace deals that ended the war were supposed to move Liberia past its recent bloody history.
At least 200,000 people died during the civil war from 1989 to 2003 that devastated the country.
International election observers said that the two rounds of voting had largely been clean, and the national elections council ruled that Weah's claims of ballot-box stuffing were groundless.
With heavy support among the young, jobless males that fought in Liberia's wars, Weah's continuing protests risk returning the country to strife. There are 15,000 UN peacekeepers in Liberia.