* Sudanese forces beat protesting doctors
* Many doctors injured during peaceful march, begin strike
* Security says protest without permission illegal
By Opheera McDoom
KHARTOUM, June 3 (Reuters) - Sudanese doctors said they were going on strike on Thursday after security forces attacked a peaceful protest march through Khartoum by hundreds of medical personnel.
In what was seen as crackdown on dissent following national elections, security officials armed with sticks attacked the doctors, injuring many of the more than 500 who had joined Wednesday's demonstration to protest at the arrest of their leaders, witnesses said.
Sudan's first open elections in 24 years in April were boycotted by much of the opposition and returned ruling parties in the south and north with massive majorities.
The election is seen as a precursor to a referendum next year, when people in the oil-producing south are expected to vote to secede from the north.
"I was beaten with sticks and it was chaos, everyone started running everywhere," said doctor Mayada Abdallahi. "There were about 500-600 of us."
"There was one man whose clothes were soaked in blood and he had stitches in his head," said another doctor, Rania Abdelazeem.
The doctors called off a partial strike over pay and conditions before the elections once the government agreed to some of their demands.
But after the doctors' leaders were arrested and little was implemented they decided to hold a meeting to discuss further action, but were forbidden to do so by the security forces.
"We have now decided to enter into a full, open-ended strike until our leaders are released," said psychiatrist Nahid Mohamed.
She said no doctors would be working in government hospitals throughout Sudan.
"Any gathering without permission is illegal," said a security source. "Even if it was a peaceful demonstration, they broke the law -- we cannot allow chaos."
Sudanese doctors and lawyers were the driving force behind a popular uprising in 1985, which overthrew then leader Jaafar Nimeiri.
Also on Wednesday, Sudanese security forces released one of four staff from the opposition al-Rai al-Shaab newspaper, which it had raided and closed down last month. The other three were charged with crimes including waging war against the state and espionage, their lawyer said.
"We are asking them to be taken to court," said opposition lawyer Kamal Omer on Thursday. He said Islamist opposition leader Hassan al-Turabi, whose Popular Congress Party ran the paper, had been in poor health since he was arrested last month.
"He has the right under the law to be told what the charges against him are yet he has not been questioned or charged with anything," Omer said. (Editing by Giles Elgood)