More than 100 technology companies have written to the US Federal Communication Commission (FCC), opposing potential changes to net neutrality rules.
The FCC is considering allowing internet service providers (ISPs) to charge content providers to prioritise their traffic.
Google, Facebook, Twitter and Amazon warn that such a move represents a "grave threat to the internet".
One FCC commissioner has called for a delay to the vote, due on 15 May.
Net neutrality - the premise that all internet traffic should be treated equally - has been a cornerstone of the web for many years.
But with the growth of bandwidth-hungry services such as Netflix, ISPs have increasingly asked for the right to charge a fee for carrying such data at high speed on their networks.
A landmark court case in February, in which Verizon successfully challenged the FCC's right to stop it charging such fees, pushed the regulator into a major rethink of its rules.
Rather than enshrining the principle of net neutrality, leaked reports suggest the FCC will allow ISPs to strike such deals as long as they act "in a commercially reasonable manner".
In the letter, tech companies, ranging from three-person start-ups to the biggest names on the web, made clear their dismay at the feared U-turn.
"We write to express our support for a free and open internet," it said.
"The innovation we have seen to date happened in a world without discrimination.
"Instead of permitting individualised bargaining and discrimination, the commission's rules should protect users and internet companies on both fixed and mobile platforms against blocking, discrimination and paid prioritisation."
The letter urged the commission to "take the necessary steps to ensure that the internet remains an open platform".
More than a million people have signed petitions to the FCC calling for it to abandon plans to allow a tiered internet.
A handful of protesters have gathered outside FCC headquarters in Washington, promising to camp there until the 15 May vote.
And a group of net firms and civil liberty groups have called on the FCC to reclassify broadband companies as "telecommunication services", which would give it the authority to impose net-neutrality rules on them.
One of the FCC's four commissioners, Jessica Rosenworcel, has called for the vote to be delayed by at least a month.
She said: "Rushing headlong into a rulemaking next week fails to respect the public response".
But an FCC spokesman said the vote would go ahead as planned.
"Moving forward will allow the American people to review and comment on the proposed plan without delay, and bring us one step closer to putting rules on the books to protect consumers and entrepreneurs online," he added.