Crowds packed into a speech by former foreign secretary Boris Johnson at the Conservative Party conference in Birmingham.
Mr Johnson renewed his attack on Theresa May's Brexit plan, describing it as a "cheat" that could lead to a boost for the far-right.
In a wide-ranging speech, he also called for tax cuts and an increase in house-building.
Long queues formed outside the event more than an hour before he began.
He started with a self-deprecating joke about the chancellor's Brexit warnings.
Philip Hammond's prediction that Mr Johnson would never be prime minister, he said, could be "the only Treasury forecast in some time to have a distinct ring of truth".
Mr Johnson said he wanted to "put some lead in the collective pencil" and end a "seeping away of our self-belief".
He also spoke up for police use of stop-and-search.
Corbyn, housing, praise for Thatcher, this is what we would euphemistically call a ‘wide ranging speech’ - a blatant pitch for leadership by Johnson, right in heart of conference where May meant to be in charge— Laura Kuenssberg (@bbclaurak) October 2, 2018
But Brexit was his main focus as he called for the government to ditch the Chequers plan that led to his resignation from the government in July.
He denounced the proposals - at one point suggesting the PM risked being prosecuted under a 14th century law saying that "no foreign court or government shall have jurisdiction in this country" - describing it as an "outrage".
Did the speech justify the hype?
The BBC's political correspondent Alex Forsyth in Birmingham
Expectations couldn't have been higher. Delegates started queuing at least an hour before the doors opened in order to get a seat at the much-anticipated event.
At the foot of the stage there was a press scrum, cameras trained on former Brexit ministers David Davis and Steve Baker in the front row - alongside Stanley Johnson who'd come to see his son.
No surprise then that when Boris Johnson took to the stage many in the audience rose to their feet. The speech was typically verbose and littered with jokes.
There was applause for his criticism of the Labour Party and his fierce defence of Conservative values. But undoubtedly the biggest reaction came from his attack on the Prime Minister's Brexit proposals; cheers and enthusiastic clapping as he urged the party to "chuck Chequers".
This wasn't a speech littered with standing ovations; matching the hype of such an event is hard.
But Mr Johnson - and crucially his opposition to Theresa May's Brexit plan - proved hugely popular with many in the hall today and no doubt No 10 would have been watching.
Remaining "half-in half-out" would prolong "this toxic tedious business", he said.
"If we cheat the electorate - and Chequers is a cheat - we will escalate the sense of mistrust.
"We will give credence to those who cry betrayal, and I am afraid we will make it more likely that the ultimate beneficiary of the Chequers deal will be the far right in the form of UKIP."
After the speech, he left the venue in a media scrum as he faced questions about whether he had been setting out a leadership pitch.
Scottish Secretary David Mundell said people were "concerned about Mr Johnson's behaviour".
Brexit negotiations are at a "really critical point" and people should support the prime minister, he said.
"Unfortunately Mr Johnson seems to behave in a way that suggests that he's only focused on his own self-interests and not on the interests of our country."
Earlier party chairman Brandon Lewis brushed off claims Mr Johnson's alternative agenda sought to undermine Theresa May.
"It's always dangerous getting into hypotheticals about what people may or may not say in the future, even if it's just later on today," he said.
Mr Johnson has been a vocal critic of the prime minister's Brexit plan since leaving the Cabinet, calling it "a moral and intellectual humiliation" when setting out an alternative last week.
Ahead of his conference appearance, he was pictured out jogging, in what some interpreted as a "parody" of Mrs May's claim that the "naughtiest" thing she did as a child was to run through a field of wheat.
When asked before the fringe event how she had been getting on with Mr Johnson, the prime minister laughed and told BBC One's Breakfast: "Well, I'm sure that's going to be a very lively event."
Questioned again later on BBC Radio 4's Today about how she felt that her former colleague appeared to be mocking her, she replied: "What I feel is that I, and this government, and this party, are getting on with the important job... of getting a good deal for the United Kingdom when we leave the European Union."
Mr Johnson's attitude towards business - he was reported to have used the f-word in response to their Brexit concerns - was criticised by Sir Digby Jones, former boss of lobby group the CBI, at the weekend.