No official toll was available on Sunday evening, but AFP counted at least five men with injuries, some with large gashes on their heads.
Supporters of former football star Weah's political group, the Coalition for Democratic Change (CDC), and members from the opposition ALCOP, threw stones at each other, and in response, police fired tear gas.
The main political parties have pledged peaceful polls, but the deaths of three people in clashes last month between rival political camps have fuelled fears of electoral violence in the country, which suffered back-to-back civil wars between 1989 and 2003 that left more than 250,000 people dead.
Peace and development have been listed among voters' top priorities before they head to the first round of voting on Tuesday.
To win, a candidate must get at least 50 percent plus one of the votes cast.
If no party reaches that threshold, the two parties with the most votes in the first round will proceed to a run-off election that is won by a simple majority.
Weah, who is up against 19 presidential candidates, gathered thousands of people in the capital on Sunday for the final act of his campaign.
"We must all cherish this peace and continue to preserve it, because without peace, our world will be difficult. Without peace, development will not take place," he said in a speech at his party's headquarters.
Weah came to power promising to create jobs and invest in education, but critics say he has failed to meet his pledges.
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