ISLAMABAD, June 26 (Reuters) - Pakistan's army has sacked three senior officers, including a lieutenant general, for failing to prevent violent attacks on military assets by ex-prime minister Imran Khan's supporters protesting his arrest, the army's spokesperson said on Monday.

It was a rare public announcement by the army of an internal inquiry and its outcome.

At least 102 people are on trial in military courts over last month's violence, Major General Ahmad Sharif Chaudhry told a press conference in the garrison city of Rawalpindi.

Chaudhry gave no details regarding how many of them were civilians or military officials. He also declined to name the senior officers who had been fired.

Human rights groups have raised concerns about military trials of civilians in Pakistan that they say cannot ensure a fair defence. The trials have also been challenged in Pakistan's Supreme Court in three petitions, including one by Khan's party.

In May, thousands of Khan's supporters rampaged through military installations across the country and vandalised them, including an air base, several military garrisons, the house of a general and the army's headquarters. Over 5,000 of them were arrested, though most were later released.

"We had to determine why security was breached at army installations. We had to find out what had gone wrong," Chaudhry said.

He said two departmental inquiries were conducted, headed by major generals, and punishments were given according to their recommendations.

Strict departmental action had also been taken against another 15 army officers, including three major generals and seven brigadiers, Chaudhry said, as part of internal accountability in the military. He did not specify what action had been taken.

Chaudhry added that several relatives, including women, of senior army officers were also facing trials for allegedly being facilitators of the violence.

The army has said the arson was pre-planned by leaders of Khan's party, and have named him in at least two criminal cases as abetting the violence.

Khan, 70, a former international cricket hero turned politician, has faced a slew of cases since he was ousted from power in a vote of no confidence last year, which he blames on the military's generals, a charge the army denies.

Khan's party has been subjected to a massive security crackdown since the May 9 violence.