Jewish groups have boycotted the Argentine government's Holocaust Day commemoration over the mysterious death of prosecutor Alberto Nisman, as scepticism grows over President Cristina Fernandez's plan to reform the intelligence services she suggests are behind his death.

The annual commemoration, normally co-hosted by the government and several Jewish groups, was held at the Foreign Ministry in Buenos Aires.

A government statement said a representative of the Israeli embassy attended the official event, but it was shunned by local Jewish leaders.

Earlier in the day, about 500 people gathered for a ceremony at a Jewish community centre that was bombed in 1994, killing 85 people and injuring more than 200.

Mr Nisman was investigating that attack and had accused the president of plotting to shield Iranian officials blamed for the car bombing.

"In honour of the victims, we are not going to share a table with those who don't give us the truth, much less justice," Mario Comisarenco, treasurer of the Delegation of Jewish-Argentine Associations, said during a ceremony that involved prayer and lighting candles.

Mr Nisman was found shot dead on January 18, the day before he was to give evidence to congress to elaborate on his accusations that Ms Fernandez helped Iran's government cover up its alleged involvement in the attack in exchange for favourable economic deals.

Iran has repeatedly denied any role in the attack, and Ms Fernandez rejected Mr Nisman's claims.

In an hour-long televised speech on Monday night, Ms Fernandez called for congress to overhaul Argentina's intelligence services. She plans to present a reform bill by the end of this week.

She said only a few government members would have access to the heads of a new "Federal Intelligence Agency", apparently a critique of a system where many in congress have contact with intelligence officials.

Opposition leaders rejected her proposal, and noted Ms Fernandez already had oversight of the country's spy agencies.

"The Intelligence Department is not going to change with a modification of its name," said Buenos Aires mayor Mauricio Macri, who is considered a leading potential presidential contender for October elections. "The change will happen if power is exercised differently."

In her speech, Ms Fernandez did not say who killed Mr Nisman, but suggested rogue intelligence agents might have orchestrated his death in a plot against her government.

Mr Nisman's death has particular resonance in the Jewish community, one of the largest outside Israel. Many fear it makes it less likely that families of bombing victims will ever get justice.

While government officials previously labelled Mr Nisman's allegations of a cover-up absurd, Ms Fernandez's speech was the first time she took them on directly.

"It's unreasonable to think our government could even be suspected of such a manoeuvre," she said, sitting in a wheelchair because of a broken ankle.

Mr Nisman's death has produced anti-government protests and myriad conspiracy theories, ranging from suicide to the involvement of Iranian intelligence agents.

Ms Fernandez noted in her speech that her predecessor as president, her late husband Nestor Kirchner, appointed Mr Nisman to the case after years of paralysis.

Authorities said Mr Nisman's remains would be handed over to his family today and buried tomorrow.