GAZA, March 19 (Reuters) - Several armed men entered Reuters' office in Gaza on Saturday, threatened employees with guns and took away a video camera, apparently after they spotted a reporter filming a demonstration from the building.

The men struck one Reuters journalist on the arm with a metal bar and threatened to throw another out of the window of the high-rise block. The group, which numbered about 10 men, smashed a television set and other equipment before leaving.

The same group, several of whom were carrying pistols, also forcibly entered the nearby offices of US broadcaster CNN and the Japanese station NHK. They seized videotape at NHK.

The men told Reuters journalists that they came from the internal security services of Hamas, the Islamist group which governs the Palestinian enclave, but they showed no documents.

A senior official of Hamas condemned the violence and denied that the group was involved in the attack:

Initial information shows these men were not from the government. We have arrested some of them and we are going to interrogate them and see who they were acting for, Interior Minister Fathi Hammad told reporters.

He added that he had told all security services to treat journalists with respect and prevent attacks on them.

Reuters Editor-in-Chief Stephen Adler said: We are extremely concerned at this unwarranted assault on our staff and urge the authorities to ensure that journalists can work freely in Gaza. The Reuters staff attacked were Palestinian.

The violence flared shortly after uniformed Hamas security forces broke up a small rally which had been called to call for Palestinian unity and reconciliation between Hamas and President Mahmoud Abbas's rival Fatah group.

Reuters had filmed a journalist from the Associated Press being detained at the protest. The man was later released, AP said.

After the attack on the Reuters newsroom, more than 100 Palestinian journalists staged a sit-in in the street in Gaza to condemn the violence and to demand a full investigation.

It was the second time in a week that media organizations have came under attack in Gaza. Witnesses said security forces beat photographers and cameramen on Tuesday as they tried to film another Palestinian unity rally in the city.

The Foreign Press Association, which represents journalists in both Israel and the Palestinian Territories, condemned the March 15 crackdown: This is the latest in a string of chilling attacks on reporters in Gaza, it said in a statement.

Hamas denied accusations that it was responsible for the violence, blaming the assault on different youth groups.

Hamas won parliamentary elections in the Palestinian Territories in 2006 and seized control of the Gaza Strip 18 months later after a brief civil war with Abbas's movement.

Fateh controls the nearby West Bank and numerous attempts to reconcile the two Palestinian factions have failed.

(Writing by Crispian Balmer; Editing by Alastair Macdonald)