Supporters carried the Romanian and Moldovan flags, which are virtually identical, and shouted: "Unity," and "Bessarabia, Romanian land," using the old name for Moldova, AP reported.
Moldova united with Romania in 1918 and was annexed to the Soviet Union in 1940. It gained independence in 1991.
Demonstrators applauded a group of several dozen Romanians who joined Saturday's reunification rally after a 1,300-kilometer (812-mile) "Centenary March" march they started in Romania on July 1.
The Romanians had been barred entry from Moldova for several days this week after border police said they were a threat to public order.