The court accepted a petition by Fonseka’s wife requesting that his arrest be ruled illegal, and ordered the state to allow the former army chief family visits and medicines.

"The court granted leave to proceed with the case," a court official told AFP.

Fonseka was arrested on Monday, two weeks after being trounced in presidential elections by the incumbent, President Mahinda Rajapaksa. His detention triggered violent protests in Colombo and other parts of Sri Lanka.

The Defense Ministry says he is to be court-martialed on unspecified charges of conspiring against the government.

A lawyer for Fonseka said they regarded the Supreme Court decision to take up the petition as a major victory.

Hundreds of Fonseka supporters packed the courtroom, while police manned barricades outside.

Anti-riot squads had been put on alert after violent clashes Wednesday outside the court complex between Fonseka loyalists and ruling party activists.

Members of the elite Special Task Force commandos were also deployed in the city.

"We have taken preventive measures to ensure that law and order is maintained," police spokesman Prashanth Jayakody said.

Hundreds of lawyers marched outside the court after the hearing carrying placards demanding Fonseka’s release, but there were no clashes.

As the court proceedings got under way, the parliamentary opposition leader and former Premier Ranil Wickremesinghe met with President Rajapaksa to push for the former army chief's freedom.

Wickremesinghe also insisted on being allowed to meet with Fonseka, who is detained at the navy headquarters in Colombo.

Fonseka's wife says her husband needs medicines on a regular basis for the injuries he received in an April 2006 suicide bomb attack blamed on Tamil Tiger rebels.

An opposition statement said Rajapaksa promised to take a decision on the fate of his former army commander, but only after military investigations into the alleged conspiracy against the government.

The government has yet to specify the charges Fonseka will face, but Defense Secretary Gotabhaya Rajapaksa — the president's brother — said he had clearly been plotting a military coup.

The arrest and detention of Fonseka has sparked international concern with the United States, the European Union and the United nations, among others, asking Colombo to ensure that due process was followed and that democracy was not undermined.

As the battlefield architect of the victory over the Tamil Tiger rebels last May, Fonseka was hailed as a national hero for finally crushing their 37-year campaign for an independent Tamil homeland.

But his bid to translate that military success into political power proved his undoing.

Fonseka, who quit the military in November, entered politics as the common opposition presidential candidate in the Jan. 26 vote, which was easily won by Rajapaksa.

Meanwhile, the United States denied accusations that it bankrolled Sri Lanka's defeated and imprisoned presidential candidate, and the US Embassy called on the government to protect the rights of opposition supporters and media.

The denial came after reported allegations by Sri Lanka's defense secretary that the US and Norway financially supported the opposition's attempt to oust the government in last month's presidential election.

“The United States backed no candidate but strongly supported a free, fair, and credible democratic process,” the US Embassy said in a statement Thursday night.

In an interview with Singapore's Straits Times newspaper published Thursday, Defense Secretary Gotabhaya Rajapaksa, the president's younger brother, was quoted as saying about Fonseka: “We are 100 percent convinced that Western countries with vested interests were backing him. Even the US, and countries like Norway, spent lots of money on his campaign.” He also said he had proof that Norway, which earlier had brokered unsuccessful peace talks between the government and rebels, had paid journalists to “write against the government.” “They have vested interests and used to support the Tamil Tigers in various ways. They also supported Fonseka to try oust the president,” he was quoted as saying.

The Norwegian Foreign Ministry denied the allegations, saying “Norway has never and will never interfere with other countries' elections or in any way try to undermine or oust a democratically elected government and president.”

The US Embassy statement called on the government to respect the rights of those who supported the opposition in the election, after reports that scores of opposition activists were detained, harassed or pressured. Media rights groups rank Sri Lanka among the most dangerous places in the world for journalists who criticize the government.

The country will hold general elections on April 8, when the ruling coalition hopes to secure an absolute majority in the country's Parliament, which would help it cement its already tight grip on power.