The Green Party in England and Wales has launched its campaign for the UK to stay in the EU, saying it will make a "loud and proud" case for membership.
MP Caroline Lucas said the EU offered protections for the environment and workers' rights, while freedom of movement rules were a "wonderful gift".
She acknowledged the EU "isn't perfect" but argued that the UK should seek a "better Europe" from within.
But not all the party agrees - Green peer Baroness Jones backs a UK exit.
Leader Natalie Bennett said the party respected the fact that some people would have different views, and insisted it would not be an issue.
The referendum on the UK's EU membership is on 23 June, when voters will be asked to decide whether they want to stay in or leave the bloc.
Ms Bennett said the vote was an opportunity to engage people - particularly the young - in politics and ensure they get themselves on the electoral register.
At the launch, campaign spokeswoman Ms Lucas said: "We'll be campaigning to stay inside the EU and using that position to change it for the better.
"We're going to be mobilising our members and supporters with a loud and proud campaign," the Brighton Pavilion MP added, promising an "unremittingly positive" message.
The campaign to stay in the EU has been dubbed "Project Fear" by opponents.
Ms Lucas said the case for staying in the EU should not be focused solely on the economics and stressed that the party would "inspire people with a vision of a different kind of EU".
The campaign will have a particular focus on young voters who, she said, were more likely to be pro-EU but less likely to vote.
Setting out the benefits, as the party sees them, of the EU, Ms Lucas said it played an important role in helping to control big business and finance, and to safeguard the rights of workers and consumers.
She also praised EU-level action to tackle climate change and improve the environment. The party says UK beaches are cleaner and the air less polluted because of EU laws.
Freedom of movement should also be "celebrated", she said, saying the ability of EU citizens to work, study and retire in other EU countries was a "wonderful gift". And she said the EU had been crucial in securing a "lasting peace" in Europe which "we forget... at our peril".
She said the organisation needed to be more democratic and accountable, but that was not a reason to walk away from it, she argued.
"To make the EU better for every European we need to stay in and reform it. Let's not leave the argument to the old political elite."
Leader Natalie Bennett said it was important to warn that an EU exit would create a "world of uncertainty".
She said the EU referendum "is a threat" but it also provided an "opportunity" to involve young people in politics and improve voter registration.
"I think we should use this as an opportunity to engage people to say 'make sure you can have your say in this referendum'," she added.