As activists from Swedish teenager Greta Thunberg to "Extinction Rebellion" movement protesters take an increasingly visible stand on climate change, officials feel growing pressure to respond, mayors gathered in Copenhagen said.

"If you want to walk the streets of some of the biggest cities in Poland, you've got to do something about climate change," Warsaw's mayor, Rafal Trzaskowski, told the C40 summit of mayors committed to action on climate change.

"The pressure especially from young people is just enormous," he said. "They are fed up with declarations, with goals and priorities ... they want to see results".

Leaders of more than 90 cities representing more than 700 million people and a quarter of the global economy met in the Danish capital this week to push for faster action to stem climate threats.

The gathering came as Extinction Rebellion climate change activists took to the streets from London to New Zealand for two weeks of peaceful civil disobedience.

"Mayors are on the frontline now," former US vice president Al Gore told the conference. "You do not have the luxury of just going away and reading reports about what is going on. You run into your constituents every single day."

On Thursday Extinction Rebellion protesters attempted to shut down London City Airport, with one protester laying on top of a plane. On Friday they blocked the headquarters of the BBC, Britain's public service broadcaster..

Earlier this week, French members of the movement blocked a street and bridge in Paris' Chatelet district.

The city's mayor, Anne Hidalgo, said she shared their concern.

"We are facing threats of climate breakdown – the response that we're seeing from Extinction Rebellion and young climate strikers around the globe should come as no surprise," she said.

In Copenhagen, city leaders unveiled a series of pledges and measures to cut planet warming emissions, from reducing meat consumption to tackling air pollution with cheap bus fares and vehicle bans.

But delegates at the summit opening Wednesday were met by demonstrators from Klima Aktion DK, a local climate action group, armed with fake binoculars made from toilet rolls.

"Our message to the C40 Mayors is: The people are watching you! We want to see action!" the group wrote on Facebook.

Read also: 'Never give up', Swedish teen tells Iowa climate activists after UN summit 'failure'

Empty words

Cities are vital to limit global warming as they account for about three-quarters of carbon emissions and consume more than two-thirds of global energy, according to the United Nations.

Wider use of existing clean technologies - such as electric buses - could deliver more than half the cuts needed to keep global temperature rise to below 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial times, the UN says.

World leaders agreed in the 2015 Paris Agreement to hold temperate hikes to "well below" 2 degrees Celsius.

"Cities are where the climate battle will largely be won or lost," U.N. Secretary-General Antonio Guterres told the Copenhagen summit.

Youth activists invited to the meeting to discuss possible climate measures demanded a bigger seat at the table and called for stronger action.

"We should be a part of the decision process when decisions regarding our future are being taken," said Selma Norgard, a young Danish climate activist.

Luisa Neubauer, a founder of the German branch of Fridays for Future, the movement started by Thunberg, said her 10 months of activism on climate change had taught her to distrust politicians.

"We have become experts in listening to empty words and to hearing empty promises over and over again. That is painful," she said.

"We just ask to grow old on a planet that can provide livelihoods for all of us, no matter where we are born".

Hilda Nakabuye, the founder of Fridays for Future in Uganda, said her family had been forced to sell of its land and livestock as heavy rains, high winds and dry spells linked to climate change had wiped away crops and made the soil barren.

"Your beds might be comfortable right now but not for long. You'll soon feel the same heat that we feel every day," she warned.

Read also: Scientists endorse mass civil disobedience to force climate action

No magic wand

Many mayors in attendance said they were inspired by the resolve shown by young activists - and somewhat surprised by the rapid growth of their movement.

Milan's mayor, Giuseppe Sala, said the city council was planning to consult young people in climate matters.

But translating ideas into concrete action was not always easy, as not everybody agrees on what should be done, said the mayor of Lisbon, Fernando Medina.

Even simple measures such as creating more cycle lanes often face opposition from residents and politicians when, for example, they encroach on parking spaces, he said.

"I don't have a magic wand," he said. "But I can tell (young people) that I can work every day to make things happen more quickly".

Others lamented that cities alone could not solve the climate crisis.

"Cities are leading the change, but countries need to do their part too," said the mayor of Barcelona, Ada Colau.

At least 70 countries announced plans at a UN climate action summit last month to boost their efforts to cut emissions, but most major economies, including the United States and China, failed to announce stronger new measures.

The world is now at a crossroads between "extinction or opportunity", said US Rep. Alexandria Ocasio-Cortez, a Democratic member of Congress.

"Every decision we make between now and (within) our children's lifetimes will determine how human life will fare, whether we'll suffer greatly or build together," she told the summit.