Twitter and the Huffington Post UK confirmed they were hacked after the Syrian Electronic Army made claims it had taken over the sites.
Twitter said the hack led to availability issues for an hour and a half but that no user information was compromised.
The Huffington Post attack was limited to the blogging platform's UK web address, a spokesperson said.
The attacks came as the Obama administration considers taking action against the Syrian government.
More top news
Ukip leader Paul Nuttall also said he would want to get rid of Sharia courts.
Over 40,000 people picked up their packs to take part in the annual marathon in the capital.
The country's voters are concerned about security, the economy, and the 10% unemployment rate.