VOGHERA, Italy (AP) _ Imprisoned financier Michele Sindona, in a coma since swallowing a massive dose of cyanide, grew weaker Friday, doctors said. Lawmakers demanded to know how he was poisoned two days after being convicted of planning a murder.

In the latest medical bulletin from the Voghera Hospital, doctors said Sindona's heart had weakened. He had lapsed into a coma Thursday and the doctors said tests showed he had no brain activity.

They said earlier that the 65-year-old Sindona had swallowed a massive dose of cyanide in his cell at Voghera Prison. He was convicted Tuesday in a Milan court of planning the murder of a bank executive and sentenced to life in prison.

Sindona was convicted in the United States of fraud in the collapse of the Franklin National Bank, America's largest bank failure. He was extradited to Italy in September 1984 for the trial in Milan.

Legislators demanded an investigation, and Justice Minister Fermo Mino Martinazzoli told the Chamber of Deputies he had received a report that a prison guard heard Sindona say when he became ill, ''They have poisoned me.''

Sindona, who was serving a 25-year sentence in the United States when he was extradited to Italy in 1984, collapsed in his cell in the prison in this northern city during breakfast.

There were conflicting reports, and investigators refused to speculate whether Sindona was poisoned or the cyanide was self-administered.

The Italian news agency ANSA said guards told investigators they believed the poison was in a glass of milk, and that after Sindona took a sip he let out a cry.

But Martinazzoli said Sindona had drunk coffee in the toilet of his cell.

Oreste Dominoni, a lawyer for Sindona's family, told reporters Sindona had been poisoned. He was asked by whom and replied, ''I cannot say ... That has to be investigated.''

In 1980, before he was sentenced in New York, Sindona took an overdose of digitalis, a heart stimulant, and an experimental antidote flown from Boston was credited with saving his life.

The justice minister, a Christian Democrat, came under fire from lawmakers of other parties who claimed security was lax at the Voghera Prison.

Several deputies criticized what they said was easy access to the prison and cited as an example an interview Sindona gave to the state-run television network from his cell the night after he was sentenced in Milan.

Martinazzoli said that after Sindona was imprisoned here, he was watched around the clock in his cell by three closed-circuit television monitors. He said breakfast was delivered to Sindona's cell in a closed metal box that could only be opened by guards with a key in the prisoner's presence.