The Labour Party says there has been a “sophisticated and large-scale cyber-attack” on its digital platforms.
Labour said the attack “failed” because of the party’s “robust” security system and no data breach had occurred.
The Distributed Denial of Service (DDoS) attack floods a computer server with traffic to try to take it offline.
A Labour source told the BBC “tens of millions of attacks – mostly originating from Russia and Brazil” had been detected.
DDoS attacks direct huge amounts of internet traffic at a target in an effort to overwhelm computer servers, causing their software to crash.
They are often carried out via a network of hijacked computers and other internet-connected devices known as a botnet, the owners of which may be unaware their equipment is involved.
DDoS attacks are not normally recognised as being a hack as they do not involve breaking into a target’s systems to insert malware.
They can vary in sophistication and size, and are sometimes used as a diversionary tactic to carry out a more damaging attack under the radar.
Several companies provide services to repel DDoS attacks, but they can be costly.
In a letter sent to Labour campaigners, Niall Sookoo, the party’s executive director of elections and campaigns, said: “Yesterday afternoon our security systems identified that, in a very short period of time, there were large-scale and sophisticated attacks on Labour Party platforms which had the intention of taking our systems entirely offline.
“Every single one of these attempts failed due to our robust security systems and the integrity of all our platforms and data was maintained.
“I would I like to pay tribute to all the teams at Labour HQ who identified this risk and acted quickly to protect us.”
The Labour Party spokeswoman said: “We have experienced a sophisticated and large-scale cyber-attack on Labour digital platforms.
“We took swift action and these attempts failed due to our robust security systems. The integrity of all our platforms was maintained and we are confident that no data breach occurred.”
It had been reported to the National Cyber Security Centre, she said.
Security procedures had “slowed down” some election campaign activities, she added, but they were now “back up to full speed”.