Mr Hwang lives in a safe house

He and another defector, Kim Duk-hong, say they have been prevented from giving lectures, writing books, issuing a newsletter and joining private democratic movements in the Stalinist North.

The allegations have been made at a time when South Korea is becoming increasingly involved in a process of reconciliation with the Pyongyang government.

Safe house

Mr Hwang was a close confidant of North Korean leader Kim Jong-il before his high profile defection in 1997.

He was a senior secretary of the ruling Worker's Party and was behind the country's ideology of "juche" or "self-reliance".

Kim Jong-il: Strongly criticised

But he is now an outspoken critic of Mr Kim's regime and lives in a safe house under the protection of Seoul's National Intelligence Service (NIS).

Mr Hwang has also criticised South Korean President Kim Dae-jung for his policy of engagement with the North.

In an article published in Japan last week, Mr Hwang said it was "tantamount to suicide" for the South to give economic aid to Pyongang's "communist dictatorial regime".

Assassination

The case has prompted the opposition Grand National Party (GNP) to call for the resignation of the NIS chief.

The NIS denies the defectors' allegations, saying they have enjoyed "free activities under its protection".

However, an NIS statement admitted there was concern that their activities were harming Korean reconciliation efforts.

"In order to protect them from possible assassination attempts and because their activities are not helpful for the advancement of inter-Korea relations, we have exhorted them to restrain themselves," it added.

Relations on the Korean peninsular, which split in two in 1945, have improved markedly since a historic summit in June.

South Korea's Yonhap news agency said the NIS had rejected requests for media interviews with Mr Hwang since the summit.