Mrs Clinton's husband, former US President Bill Clinton, was also featured
Democratic presidential hopefuls in the US have clashed over several issues during a TV debate.
The debate, in Charleston, South Carolina was hosted by NBC and YouTube.
Former Baltimore Mayor Martin O'Malley, Hillary Clinton and Bernie Sanders started by explaining the top three priorities for their first 100 days in office:
Democrats Outline Their First 100 Days As President
Watch all 3 Democratic candidates complete the statement: "My first 100 days in office, my top 3 priorities will be..."Catch all the highlights, viral moments and analysis from the debate here: http://nbcnews.to/demdebateblogPosted by NBC News on Sunday, 17 January 2016
On the devisive issue of gun control, Mrs Clinton said America needs to put families before the gun lobby.
Mr Sanders countered, sayinh he would support stronger provisions on gun restrictions.
Mr O'Malley talked also about his policies, which critics have said led to the unrest in Baltimore after the death of Freddie Gray while in police custody.
O'Malley: Black Lives Do Matter
'YES, BLACK LIVES MATTER'Martin O'Malley responds to critics who say his "zero tolerance" stance on crime as Baltimore mayor indirectly led to some of the unrest in his city following the arrest of Freddie Gray.Catch all the highlights here: http://nbcnews.to/demdebateblogPosted by NBC News on Sunday, 17 January 2016
Mrs Clinton says there needs to be "a concerted effort" to address "systemic racism" in the US justice system.
On the issue of attacking Mrs Clinton's husband, former US President Bill Clinton, Mr Sanders said he is running "an issue-oriented campaign" and would not be addressing the question.
The two also traded barbs on US healthcare.
Latest polls show Mr Sanders in a dead heat with Mrs Clinton in Iowa, where the caucuses for both Democrats and Republicans will take place at the start of February.
However, Fortune magazine says: "It's doubtful that either Clinton or Sanders scored a decisive victory in the minds of enough voters to tip the balance of a race that remains nip-and-tuck, at least in the first two states".
Don't have time to watch all that? Catch-up on the debate in this two-minute breakdown.
Here's what you missed in Sunday's NBC News YouTube Democratic debate — in two minutes.Posted by NBC News on Sunday, 17 January 2016