Reuters) - Separatists attacked a base of the Ukrainian national guard in an eastern city overnight and Kiev said three attackers were killed in the shooting that followed, making it the bloodiest incident so far in a 10 day uprising in the east.

A single grey police jeep was inside the compound on Thursday morning with broken windows, flat tires and bent doors. The gates of the compound had been flattened. There were shell casings outside the gates and several unused petrol bombs.

"They came here around 8:15 p.m. (Wednesday), demanding that we surrender our weapons and join the people. There were some women with them, but then they left," said police major Oleksandr Kolesnichenko, deputy commander of the base.

"Then they used a truck to break through the gate. There was some incoming fire, I could not see who was shooting it was dark," he said. "We fired first in the air. We fired warning shots after they entered the compound. We had no casualties. We are safe."

Heavily armed pro-Russian separatists have seized buildings in about 10 towns and cities in eastern Ukraine. Moscow has massed tens of thousands of troops on the border and says it has the right to intervene to protect Russian speakers if there is any violence.

A separatist representative, who gave his name only as Sergei, said there had been a peaceful rally at the base.

"We had a peaceful rally to urge the police to join the people. The commander of the compound warned he would order troops to shoot to kill."

"Then there was shooting. Some people came with Molotov cocktails. We have verified that one person is dead and more than 10 wounded."

Ukraine's interior minister Arsen Avakov said in a post on Facebook that an armed group of about 300 people had attacked the base in Mariupol on the Sea of Azov with guns and petrol bombs. National guard members first fired warning shots but then opened fire as the attack continued, Avakov said.

"According to preliminary data, three attackers were killed, 13 wounded and 63 detained," he said.

No guardsmen were killed, Avakov said, but a major security sweep was underway in the area to detain other armed members of the group.

"Given the aggressive nature of the attack on the base, an interior ministry group has been strengthened by Omega special forces. Helicopters have been sent in," Avakov said. Arms had been seized from the attackers as well as communications equipment, he said.

The Mariupol city administration was seized at the weekend by separatists.

Previously, two people have been confirmed killed in the 10 day uprising, including a member of the Ukrainian security forces. Russian media have previously issued reports of deaths among pro-Russian separatists that turned out to be false, although unlike the latest incident in Mariupol those were not confirmed by the authorities in Kiev.

Alexander, 71, who lives nearby, said: "The shooting lasted between 10 p.m. and 2 a.m. I've heard firing from the base, then two helicopters came, hovering overhead for some time, then I saw paramedics in vans trying to get into the area. I did not hear firing from the crowd, only saw Molotov cocktails."

(Additional reporting by Aleksander Vasovic in Mariupol; Writing by Richard Balmforth)