National security judges began hearing closing submissions on Monday in the trial of former media tycoon Jimmy Lai, with prosecutors laying out arguments on key legal elements of the case.

Lai had pleaded not guilty to three conspiracy charges relating to colluding with foreign forces and publishing seditious material.

The start of final arguments had been delayed twice, due to bad weather and the need to address the defendant's medical needs.

The bench of three judges earlier adjourned proceedings and ordered that Lai be given medication and a cardiac monitor as recommended by a doctor, after the court heard the defendant was experiencing heart palpitations.

Prosecutor Anthony Chau confirmed that Lai was provided with the items on Friday, and the defendant raised no complaint over his health condition.

One of the judges, Esther Toh, acknowledged a medical assessment which deemed Lai to be fit for court, saying additional breaks could be arranged.

In closing submissions, Chau addressed a number of legal issues such as what the offence of "conspiracy" entails.

The charge of conspiracy would hold so long as the parties involved made an agreement and adhered to it, he argued.

Lai's lawyers previously cited a civil law concept and contended any agreement made before the national security law took effect should be seen as being "frustrated".

Responding to the defence's contention, Chau argued in his submission the doctrine from contract law was inapplicable to a criminal conspiracy, and that continuing the conspiracy agreement after the security law's implementation constitutes an offence.

Prosecutors are expected to take two days to make closing arguments.