Dominican Revolutionary Party (PRD) leader Miguel Vargas blamed the incident on former president Hipolito Mejia, who denied any responsibility.
Mr Mejia was expelled from the PRD earlier this month for insubordination.
The two men have been fighting over the leadership of the party for more than a year.
Mr Vargas accused Mr Mejia of hiring "a mob", which he said fired the shots in an attempt to take over the party headquarters.
Mr Vargas had convened a meeting of the party's executive for Sunday morning, warning Mr Mejia and three other politicians, who have also been expelled, to stay away.
A group of people stormed the meeting and television pictures showed men hitting each other with plastic chairs while another man could be seen limping away with a gunshot wound to his leg.
Police spokesman Maximo Baez said the security forces had retaken control of the PRD headquarters and would investigate the incident.
Mr Mejia, who governed the country from 2000 to 2004, arrived at the party headquarters shortly after the fighting.
He said the police investigation would reveal who was behind the incident.
Mr Mejia ran for president in May 2012 but lost to Danilo Medina of the Dominican Liberation Party.
He blamed his loss on Mr Vargas, accusing him of having formed a pact with Mr Medina.
Mr Mejia, for his part, has accused Mr Vargas of trying to oust him as party leader.
UPDATE from DR1.com
Hipolito hands over PRD House to Police
After Hipolito Mejia's followers, in the midst of a shootout with Miguel Vargas's followers, took over the PRD national headquarters, Mejia, a former President of the DR, reached an agreement with government officials and the National Police occupied the opposition party's main offices.
When things cooled down and the shooting stopped, PRD general secretary Orlando Jorge Mera reported that Mejia had reached an agreement with the Chief of Police, Jose Armando Polanco Gomez and the ministers of the Presidency, Gustavo Montalvo and Interior and Police, Jose Ramon Fadul, for the police to take over the building until the conflicting factions reach an agreement. "They have committed to maintaining the building, without handing it over, until an agreement within the PRD is reached," proclaimed Jorge Mera, amidst shouts from militants who oppose the deal.
In the face of the resistance of his allies, Rafael (Fello) Subervi intervened and said that Mejia had warned the government officials and the Chief of the Police that he would only hand over the premises if they guaranteed that it would not be returned to Vargas. After this was made clear, under protest, the group left the headquarters and the PN took over the building.
Leonel slams PRD for fighting on Duarte Day
Former President Leonel Fernandez criticized the PRD without mentioning it by name. He indirectly accused the opposition party of showing a lack of respect for the events commemorating the bicentennial the birth of Duarte, the nation's founder.
"What a contrast is seen today in the Dominican society, while the PLD family gathers here, raising the tricolor flag and honoring the memory of the immortal Father of the Nation, there are others conspiring against the health of the nation," said the former President while heading a ceremony at the Altar to the Homeland in his post as PLD president yesterday, Sunday 27 January.
Fernandez said he regretted that the PRD was not capable of coming together in a civilized manner and "therefore they are eliminated from leading the destinies of the homeland forged by the ideals of Duarte, Juan Bosch, Ramon Matias Mella and Francisco del Rosario Sanchez."