Labour is promising to base a network of small business advisers in Post Office branches if it wins next Thursday's general election.
The party says the advisers would form part of a wider agency to help firms access advice and bid for government contracts.
The party says it would also help small firms by replacing business rates with a tax based on land value.
But the Conservatives said Labour would bring higher taxes and uncertainty.
The Tories have pledged to reduce business rates for smaller firms, and give them a bigger discount on National Insurance payments.
Labour's shadow business secretary Rebecca Long-Bailey said smaller companies are "being stretched to breaking point by global corporations which evade their taxes and fail to pay their suppliers on time".
"Labour wants business support and finance to be available for entrepreneurs from the moment the seed of an idea is planted," she said. "Labour's Business Development Agency will create thriving businesses within our communities, bringing life back to local economies."
The party also plans to set up a website offering support to smaller firms, and free full-fibre broadband for every business and home by 2030.
It also says it will establish a £250bn national investment bank providing loans for businesses.
In addition, it says it would requiring government contractors to pay their suppliers on time or else face a ban from bidding for public cash.
But the Liberal Democrats said Jeremy Corbyn's pledge to renegotiate the PM's Brexit deal and put it to a referendum undermined Labour's plans to support business.
Labour has pledged to offer voters a choice between its deal or remaining in the EU - it has not said which option it would back and Mr Corbyn has said he would stay "neutral" during the campaign.
Lib Dem business spokesman Sam Gyimah said smaller firms have "made it abundantly clear that any form of Brexit - be it red or blue - will harm their ability to hire staff, make it more difficult to export to our closest partners and ratchet up the cost of doing business".
"It is only the Liberal Democrats who will stop Brexit and bring forward a bold vision to support small businesses in the UK," he added.
His party also wants to replace business rates with a levy on commercial properties based on land values, and create a new "start-up allowance" to help those setting up businesses with their living costs.
The Federation of Small Businesses said it welcomed Labour's plan for an agency to support small firms, as well as the party's commitment to clamp down on suppliers that make late payments.
However its chairman Mike Cherry said the party needed to provide "urgent clarity" on its tax changes to dividend payouts.
"The party promised that no business owner making less than £80,000 would be targeted if it wins power," he said
"But, as things stand, it's hard to see how that will be the case."
The Conservatives also criticised Labour plans to raise the corporation tax rate paid by smaller companies from the current 19% to 21% by 2023/24.
The party also said Labour plans to introduce a 32-hour working week within ten years would "hit businesses hard".
International Trade Secretary Liz Truss said: "Despite what they claim, Labour are not on the side of small businesses".
She added that smaller companies "don't need a new quango, they need certainty".
"All Corbyn's Labour will bring is higher taxes and uncertainty with no plan for Brexit".
The Liberal Democrats' business spokesman Sam Gyimah said: "Labour under Jeremy Corbyn has dropped any pretence of being friendly to industry, returning to plans from the 1970s to take over company shares and nationalise swathes of the economy."
He also accused both Labour and the Conservatives of being united by Brexit, "the most anti-business policy of all".