"Although no new civil defamation suits were filed by government leaders, this practice continued to be misused to curb the right to freedom of expression and the right of political opponents to participate freely in public life," Amnesty said in its annual report.

"An array of restrictive legislation also remained in place, undermining the rights to freedom of expression and assembly," the London-based human rights group said.

Amnesty also noted the continued use of the death penalty in the city-state.

"At least five death sentences were reportedly passed during the year and 21 people were executed by hanging, 17 of whom had been convicted of drug trafficking," Amnesty said.

The death penalty is mandatory in Singapore for drug trafficking, murder, treason and certain firearms offences.

The government released for the first time statistics on execution in Singapore, revealing that 340 people were executed between 1991 and 2000.

This gave the country of 3.1 million people "possibly one of the highest execution rates in the world, relative to its population," Amnesty said.

"Caning, which constitutes cruel, inhuman or degrading punishment, remained mandatory for some 30 crimes, including attempted murder, rape, armed robbery, drug trafficking, illegal immigration offences and vandalism," it said.

Authorities also imprisoned at least 29 conscientious objectors to military service last year, Amnesty said. All of them were members of the Jehovah's Witnesses religious group.

Amnesty said the ruling People's Action Party, which has been in power since 1959 and holds 80 of the 83 seats in parliament, passed an act in May which sets tight limits on anonymous donations to political parties.

The act also prohibits civil society organisations from receiving foreign funding.

"Opposition leaders criticised the new legislation, saying it would make it more difficult for Singapore's small opposition parties to raise funds," it said.