Atalay said those detained had the same last name as the people who have been killed, pointing to intra-clan violence.
He ruled out suggestions that the PKK, a Kurdish separatist group which is active in the mainly Kurdish area where the attack took place, had been involved.
Atalay said 44 people were killed in the attack, while authorities earlier had put the toll at 45.
The wedding was held in the village of Bilge about 20km from the city of Mardin, in a region where tribal ties and rivalries sometimes eclipse the power of the state.
Before the arrest of the eight suspects, there were speculations that the attack was a result of a feud between rival groups of Village Guards – a controversial state-backed militia set up to combat Kurdish separatist fighters in southeast Turkey and provide intelligence.
"There were a few people, they broke into the house and started spraying the place with bullets, hitting both men and women, their faces were covered with masks"
Unnamed witness of the shooting
Local media said the families of both the bride and the groom included members of the Village Guard.
Al Jazeera's Anita McNaught, reporting from Istanbul, said: "Even by the standard of Turkey's intermittently violent southeast, this is a massacre of a scale they haven't seen for years if not decades.
"It's a very complex situation. One factor is the blood feud. The other factor I think we have at play is the Village Guard.
"Some of them still do the government's bidding of keeping law and order in the southeast. Others have mutated into in a quasi-mafia organisation. They're heavily armed and to degree they're running their own affairs.
"Adding to this a toxic mixture of domestic violence and high unemployment, you get the kind of climate which creates the kind of violence we're seeing."
Television broadcasters said there had been a blood feud between two families in the small village in recent years.
'Rifles and hand grenades'
Ahmet Ferhat Ozen, the acting governor of Mardin, told the Reuters news agency that masked men stormed into the hall where wedding guests were assembled and opened fire with automatic rifles and hand grenades.
An unnamed witness told Reuters how the attackers sprayed the wedding hall with gunfire.
"There were a few people, they broke into the house and started spraying the place with bullets, hitting both men and women, their faces were covered with masks," the 20-year-old woman said.
She said there were about 200 people at the wedding party.
At least six people were wounded in the attack. The bride, the groom and the groom's mother and sister were killed, according to the Associated Press news agency.
The village head of Bilge, Hamit Celebi, and 10 family members were also among the dead, Turkey's Anatolian news agency said.