Barack Obama rejected criticism on Thursday that he has delayed too long on gun reform, allowing memories of the Newtown shooting to fade and allowing time for opponents to regroup.

In a speech at the White House, attended by parents of gun victims, he said bluntly that the United States should be ashamed if the horror of Newtown was already being forgotten.

The president's comments were part of National Day to Demand Action, organised jointly with Mayors Against Illegal Guns, the campaign to put pressure on Congress to pass "commonsense" reforms driven by New York mayor Michael Bloomberg.

There have been complaints from gun reform activists that Obama has squandered an opportunity for action in the immediate aftermath of the Newtown shooting in December.

But Obama disputed that the country had moved on from Newtown. "Less than 100 days ago that happened. And the entire country was shocked, and the entire country pledged we would do something and this time would be different. Shame on us if we have forgotten. I have not forgotten those kids. Shame on us if we have forgotten," he said.

Obama said he expected votes in the Senate on gun reform measures soon after Congress returns from its Easter break on 8 April. The relatively modest package of measures he proposed in January has already been scaled back, with a ban on military-style automatic weapons effectively scuppered and even background checks to keep guns out of the hands of those with mental problems in the balance.

One of the criticisms of Obama is that instead of asking vice-president Joe Biden to oversee a task force looking at proposals for reform in January and then leaving Congress to come up with a draft bill, he should have pushed his own set of proposals when emotions were still raw.

The president at the White House, accused opponents of reform of trying to run down the clock. He called for members of the public who back gun reform to put pressure on their members of Congress, most of them back in their districts and home states for the Easter break.

Among those in the audience at the White House were the families of Newtown victims Grace McDonnell, Lauren Rousseau and Jesse Lewis, as well as the mother of Hadiya Pendleton, who was killed in Chicago, and Nardyne Jeffries, the mother of Washington shooting victim Brishell Jeffries.

Obama acknowledged he had read an article "in the news just the other day wondering has Washington missed its opportunity, because as time goes on after Newtown, somehow people start moving on and forgetting"

This was not the case, he said. "Let me tell you, the people here, they don't forget. Grace's dad is not forgetting. Hadiya's mom hasn't forgotten. The notion that two months or three months after something as horrific as what happened in Newtown happens and we've moved on to other things, that's not who we are."

It was an extremely emotional event, with people behind him, some of them families of victims, weeping throughout. "We need everybody to remember how we felt 100 days ago and to make sure that what we said was not just a bunch of platitudes. That we meant it," Obama said.

"Tears are not enough. Expressions of sympathy are not enough. Speeches are not enough. We have cried enough. We have known enough heartbreak. What we are proposing is not radical. It is not taking away anyone's gun rights. It is something if we are serious we will do. Now is the time to turn that heartbreak into something real."

Several Republicans have threatened to filibuster gun reform measures in the Senate. Without 60 of the 100 senators needed to break a filibuster, almost all of the packages proposed by Obama could fall. Obama reiterated he still wanted a ban on military-style assault weapons, even though the Democratic leader in the Senate, Harry Reid, has effectively said it has no chance.

Democrats see the best chance of getting something into law as background checks to ensure guns are not sold to the mentally unstable and checks at gun shows to ensure someone does not have a criminal background. Republicans favour increased security at schools, the line backed by the National Rifle Association.

Mayors Against Illegal Guns put out its first ad in its $12m television campaign Thursday with interviews with family members of those killed at Newtown. The ad campaign is targeting 13 states during the recess.

Bloomberg said: "We cannot afford to wait for another tragedy – it's long past time for elected officials to listen to their constituents and pass reforms like comprehensive background checks that we know will save lives."

His co-chair on Mayors Against Illegal Guns, Thomas Menino of Boston, echoed this. "The Senate now has the opportunity to take a vote that the American people are demanding to make our neighborhoods safer. The time has come to pass legislation and make reform a reality."

The coalition is hiring dozens of organisers and opening campaign offices in 10 states. More than 140 events were scheduled for Thursday in 29 states. © Guardian News and Media 2013