In a wide-ranging address to the Fine Gael Ard Fheis, Simon Harris issued a blunt message to Israeli Prime Minister Benjamin Netanyahu, saying that Ireland is "repulsed" by his actions.
"In Gaza, we see a humanitarian catastrophe worsen before our eyes," he said in his first Ard Fheis speech as party leader, condemning the threat of famine as a "spectre no Irish person can bear".
"Anyone who can countenance deliberate starvation has lost their humanity.
"Prime Minister Netanyahu, the Irish people could not be clearer. We are repulsed by your actions. Cease fire now and let the aid flow safely."
The comments drew a standing ovation from the audience of roughly two thousand party delegates at University of Galway.
The forthcoming local and European elections will be "a choice between opportunity and opportunism", Mr Harris told the delegates.
He also vowed that, under his leadership, young people will be able to "move out of the box room and into a home of their own".
Mr Harris told young people: "I want you to know your future is here in Ireland".
Ahead of his likely election as taoiseach in the Dáil on Tuesday, he said that "time is short, but there is so much to do".
One priority is to "fix the housing crisis for once and for all", he added.
'Hit the ground running'
Mr Harris took aim at Sinn Féin, who he said "promises utopia but specialises in U-turns".
The main opposition party "exploits every challenge to their own gain," he claimed, accusing the party of "pretending to support home ownership" while proposing "to abolish every housing support we offer".
He said that Sinn Féin "claims to be on the side of gardaí, but honours their killers and collects them from the prison gates," to loud applause.
Mr Harris also paid warm tribute to his predecessor, Leo Varadkar, saying that he was "deeply honoured" to take on the "greatest challenge" of his life.
"Tonight, I promise you, we're going to hit the ground running."
Outgoing Taoiseach Leo Varadkar said that he "did what needed to be done", but that it is time for the "next chapter" for Fine Gael.
Ahead of his departure from the office on Tuesday, he said he believes that Fine Gael can secure an historic fourth term with Simon Harris at the helm, and defended his party's record on housing as having delivered "real progress".
"Now is the time for the next chapter under Simon Harris, a man who has the energy, empathy, experience and campaigning skills to bring our project to the next stage."
Fine Gael will 'stand for law and order'
Insisting that Fine Gael will always stand for law and order, Simon Harris also promised new measures to increase penalties for knife crime and anti-social behaviour, including a taskforce to promote "a safer and vibrant Dublin".
Fast track legislation will give judges new powers to ensure that "heinous criminals" serve long sentences before they are considered for release.
The Government will also deliver full-body cameras and up-to-date technology to help gardaí, he said.
Mr Harris also promised to extend the rent tax credit to €1,000, and the Help to Buy Scheme for a further five years.
Fine Gael would build 250,000 homes over the next five years, more starter homes and more student housing, he added.
He reiterated his party's policy that no workers earning below €50,000 should pay the higher rate of income tax.
Later this year, Fine Gael will outline a five year tax strategy, and will "continue to work to reduce the burden of USC (Universal Social Charge) on low and middle income earners," he said.
'Energy and enthusiasm'
Minister of State for Public Health Hildegarde Naughton said that there is a "new energy and enthusiasm" in the party, and that the mood is positive.
She told RTÉ's Saturday with Colm Ó Mongáín that the party has huge talent within it.
There are "so many councillors coming up, huge talent chomping at the bit to run for election in the Dáil and we need to create the space to allow them to do that".
The party will run 330 candidates in the forthcoming local elections. In 2019, it won 255 seats, but Simon Harris would not be drawn on what the target would be in June.
Coveney to consider political future over the summer
Meanwhile, Minister for Enterprise Simon Coveney said that he does not intend to run in the European elections.
Mr Coveney said that his decision not to seek re-appointment to Cabinet was "never about paving the way to do something else, like entering European politics".
Mr Coveney expressed his belief that the party can win five seats in the European Parliament, and added that he has not yet decided whether or not to contest the next general election.
He will "take the summer to decide and to think about it, and talk to people in my own constituency".
Additional reporting by PA
