Tensions have risen on the island since former army commander Fonseka was arrested on Monday by military police on charges of engaging in politics against his president while still in uniform.

“Police are beating the protesters with batons,” said a Reuters photographer at the scene of the protest in a Colombo suburb.

Fonseka had appealed to his supporters to remain calm, his wife told reporters, after visiting the former general in detention.

"He wanted me to convey to the people and especially the troops to remain calm and not to be provoked by his illegal arrest," said Anoma Fonseka, who met with her detained husband late Wednesday.

"They are trying to break his spirit, but they won't succeed," she told reporters.

Fonseka and President Mahinda Rajapaksa worked together in ending the 25-year war against Tamil Tiger separatists last year, but fell out soon after.

The army general ran against Rajapaksa in a presidential election last month, but was roundly defeated, after which he accused his former commander-in-chief of rigging the vote.

The government said Fonseka had conspired against the president and would face a court-martial.

Police detained eight people following the protest on Thursday, spokesman Prashnath Jayakody said. "When the police asked them to move away from the main road they clashed with police," he said.

On Wednesday, eight people were injured in clashes between supporters of Fonseka and government activists.

The street protests, strikes and labor unrest could have a ripple effect on Sri Lanka's $40 billion economy, which is poised to grow over 6 percent this year due to post-war economic optimism and high foreign investments, analysts say.

Sri Lankan shares fell for a third straight session on Thursday on the growing political uncertainty, traders said.

The bench mark index was down 1.3 percent.

"Foreign investors will slow their investments looking at the present situation," said an analyst on condition of anonymity.

"They are leaving the share market, probably eyeing another emerging market. We haven't seen high post-war inflows as foreign direct investments and private equity funds as expected."

Earlier this week, Rajapaksa dissolved Parliament ahead of schedule and called elections in April, hoping to build on his own victory in the Jan. 26 presidential poll.

On Thursday, the European Union urged Sri Lankan authorities to ensure the rule of law and safety of candidates and campaign workers. The United States asked the government to protect the rights of opposition supporters and the press.

The United Nations said that Secretary-General Ban Ki-moon had discussed Fonseka's arrest with Rajapaksa in a telephone call and expressed concern about the events.