A Syrian-American pleaded not guilty in U.S. court Friday to charges he spied on anti-Assad protesters and handed recordings to Syrian intelligence in a bid to silence the opposition.

Mohamad Anas Haitham Soueid, 47, was remanded to custody by U.S. District Judge Claude Hilton, who set a March 5 trial date.

Spain summoned Syria's ambassador to Madrid on Friday, complaining of allegations that members of his embassy abused Syrian opposition sympathizers on Spanish soil, the Spanish government said.

The foreign ministry summoned ambassador Hussamedin Ala'a "after repeated complaints by Syrian and Spanish-Syrian citizens, opponents of the current regime, of acts allegedly committed by members of the embassy in clear abuse of their status," it said in a statement.

Yemenis marched by the thousands on Friday demanding President Ali Abdullah Saleh be tried, as a woman was killed by a sniper and five other people were wounded, four critically, witnesses said.

Meanwhile, an officer in charge of an anti-terrorism unit in the main southern city of Aden was killed in a car blast that was blamed on al-Qaida.

Several thousand protesters in Cairo called on the ruling military on Friday to promptly transfer power to a civilian government and exclude old regime figures from politics.

The protesters in Tahrir Square, the epicenter of Egypt's uprising that ousted president Hosni Mubarak in February, chanted "down with the military" and called on the ruling generals to "return to the barracks."

U.S. President Barack Obama on Friday congratulated Saudi King Abdullah on the selection of Prince Nayef bin Abdul Aziz as his heir, noting the new crown prince's counterterrorism record.

"I congratulate King Abdullah and the Saudi people on the selection of Prince Nayef bin Abdul Aziz as crown prince. We in the United States know and respect him for his strong commitment to combating terrorism and supporting regional peace and security," Obama said in a statement.

Lithuania said Friday that it had barred Syrian planes from crossing its airspace on their way to Russia's Kaliningrad territory amid concerns that they could transport military cargo.

A Lithuanian defense ministry spokesperson told Agence France Presse that it acted in line with European Union sanctions, which were imposed in May due to a crackdown by the regime of Syrian President Bashar al-Assad.

The head of the Islamist party Ennahda that won Tunisia's weekend elections called Friday for calm in Sidi Bouzid, the town where the north African country's revolution began, after overnight unrest.

"We call for calm and the preservation of public property," Rached Ghannouchi said after more than 2,000 youths marched on Ennahda's headquarters and sacked public buildings in Sidi Bouzid following the announcement of election results on Thursday.

The International Criminal Court is in "informal contact" with Seif al-Islam, the fugitive son of slain Libyan leader Moammar Gadhafi, its prosecutor said on Friday.

"Through intermediaries, we have informal contact with Seif," Luis Moreno-Ocampo said in a statement issued at the court's headquarters in The Hague.

An explosion at a state-run oil field in southern Iran killed a worker and injured three others on Friday, while a refinery in central Iran was hit by a fire, the Mehr news agency reported.

It said a rig at the Bibi Hakimieh oil field in the southern province of Bushehr was rocked by a blast when a huge amount of gas was released during drilling.

At least 37 people were killed Friday as Syrian security forces opened fire, encircled mosques and carried out arrests in a bid to break up anti-regime protesters who are now calling for a no-fly zone in Syria to protect civilians and soldiers deserting the army, the Syrian Revolution General Commission said.

"Eight civilians were killed in various neighborhoods of Hama, 11 others in the city of Homs and one civilian was killed in Qusayr, in the region of Homs," the Syrian Observatory for Human Rights said earlier on Friday in a statement.