Proceedings were disrupted twice by hecklers in the gallery, who were led away by police.
Scottish party leaders delivered their New Year messages ahead of the start of a new decade.
The SNP, Scottish Tory, Scottish Labour and Scottish Lib Dem leaders clashed on the BBC.
The debate will be broadcast live on STV at 8pm on Tuesday night.
Nicola Sturgeon, Jackson Carlaw, Richard Leonard and Willie Rennie went head-to-head.
The Scottish Lib Dem leader told STV's Scotland Tonight that his party wants to stop both.