SANTIAGO, Chile (CNN) -- Amid a political furor triggered by the arrest of former military dictator Augusto Pinochet, President Eduardo Frei returned to Chile Wednesday and made an open appeal for restraint.

"I want to make an appeal to the country to maintain calm and serenity and to not polarize the situation," Frei said, appealing both to Pinochet's angry supporters and his jubilant opponents.

After 117 people were arrested in violent street clashes overnight, the government announced Wednesday that it would not allow any new political demonstrations, although some small, peaceful protests were reported.

Frei was attending a summit in Portugal last Friday when British police, acting at the request of a Spanish judge, arrested Pinochet, who was in London to undergo spinal surgery. He is being detained at the clinic where he was receiving medical treatment.

The judge is seeking Pinochet's extradition on charges of genocide, murder and torture stemming from the disappearance of Spanish citizens from Chile during Pinochet's 1973-90 military dictatorship.

The Chilean government maintains that Pinochet, a senator-for-life, has diplomatic immunity. Its officials have been demanding his release, but the British government has so far refused, saying that only accredited diplomats and those on official visits are accorded diplomatic immunity.

Ambassador refuses to describe 'special mission'

In a new wrinkle to the case, Chile's ambassador to Britain on Wednesday said that Pinochet had been on a "special mission" to Britain prior to his surgery, an apparent bid to meet the requirements of British law.

However, Ambassador Mario Artaza refused to elaborate on the nature of the mission or say whether Pinochet, who was commander-in-chief of the Chilean army until last March, had been on an arms-buying trip.

Meanwhile, the U.S. government denied media reports in Britain and France that it was trying to persuade the British and Spanish governments not to pursue prosecution of Pinochet.

The reports said American officials didn't want to see Pinochet put on trial, because it might spotlight the U.S. role in the violent 1973 coup that brought him to power in Chile. But State Department spokesman James Rubin categorically denied the charge of U.S. interference.

"This is a legal matter between the governments of the United Kingdom, Spain and Chile. The United States is not involved," he said.

Reuters contributed to this report.