4chan, a site known for launching memes and harboring trolls, was down early today following an outage earlier this week that could be attributed to a digital version of a lunchroom food fight with users of Tumblr, according to a knowledgeable source.

The evidence is in what appear to be tit-for-tat online fliers circulating among users of the sites. One flier titled "Operation Overlord: Final Phase Tumblr" vowed to take down the popular microblogging site. It urged people to download the LOIC (Low Orbit Ion Cannon) software so their computers could be used to overwhelm Tumblr with traffic via a distributed denial-of-service (DDoS) attack starting November 14 at 5 p.m. ET.

"Our attacks and fear campaigns have been working quite excellently so far. Tumblr has had it too good for too long and now they are finally regretting their cancerous ways. We are trolling them hard. Now (we) just have to finally end it all and declare victory," the message said. "Now get off your asses and DDoS Tumblr!"

A rallying cry was made in a different online flier for Tumblr users to attack 4chan. "To all of Tumblr. Let us show 4chan how to really invade a site," it said. "November 15th will be the day that tumblr invades 4chan. We'll show them how it's done. Troll them using WHAT IS AIR and of course ... U MAD 4CHAN?"

Another flier, which is not dated but was circulating today, called for people to bombard 4chan's /b/ Random image board with pictures of cute kittens as part of "Operation Overkitten" to "counteract the very nature of their message board with adorableness." The message features a picture of a long white cat known as Longcat, one of the many memes popularized by 4chan users.

There were no reports of Tumblr being down yesterday, but a strike against 4Chan was successful. The site was hit with a TCP SYN flood attack that started on Sunday, according to the 4chan Status page on Blogger. This morning, that page was updated to say that the site was back up. At midday the main page took a very long time to open and the Random message board was inaccessible.

Statistics for the 4chan site compiled into a graph by Web site-monitoring firm Netcraft confirmed the outage on the 4chan message boards and, intermittently, the 4chan main page.

Netcraft notes that attacks on 4chan are nothing new and are treated with humor. A similar attack earlier this year prompted 4chan operators to quip that the site had joined "the ranks of MasterCard, Visa, PayPal et al.--an exclusive club!" Those sites were targeted with DDoS attacks by supporters of WikiLeaks late last year.

4chan founder Christopher Poole did not respond to an e-mail seeking comment from CNET. But the knowledgeable source said the attacks are "all in good fun, since after all, 4channers are used to this sorta thing."