On Friday morning, employees at Twitter's Hamburg offices were forced to step over hate-fueled messages, such as "Hitler did nothing wrong," and "Retweet if you hate Muslims," stenciled on the street in front of their office.

The stencils were plastered outside Twitter's German headquarters by Berlin-based author and satirist, Shahak Shapira, who wanted to take a stand after the social network had failed to take down any of the 300 tweets he had reported for their hateful and racist content.

Read more: Hate speech curb should look beyond Facebook, Twitter

In a video in which he explained the stunt, Shapira said: "The statements I reported weren't just plain insults or jokes, but absolutely serious threats of violence, homophobia, xenophobia or Holocaust denial. Things that nobody should say and nobody should read."

Shapira received just nine responses from Twitter to more than 300 flags. All responses said the same thing: that the content of the tweets did not violate the social network's policy on hate speech and harassment. "So I thought: 'OK, if Twitter forces me to see those things, then they'll have to see them too,'" Shapira said.

Shapira, an Israeli Jew, also grabbed headlines in January, when his 'Yolocaust' project combined photos of people posing by the Holocaust memorial in Berlin with footage from Nazi concentration camps.

Twitter's hate speech problem

According to Twitter's policies, limitations on speech are kept to a bare minimum. Users are told that they "may not promote violence against or directly attack or threaten other people on the basis of race, ethnicity, national origin, sexual orientation, gender, gender identity, religious affiliation, age, disability, or disease."

However, Twitter's efforts have not satisfied policymakers in Europe in particular. In June, the European Commission singled out Twitter and YouTube for lagging behind in complying to the EU's voluntary "code of conduct," a legislation that requires tech firms to review reported content within 24 hours.

The Commission found that Twitter only provided feedback in just over 30 percent of cases.

In a statement to the Reuters news agency, Twitter's head of public policy for Europe, Karen White, said: "Over the past six months, we've introduced a host of new tools and features to improve Twitter for everyone. We've also improved the in-app reporting process for our users and we continue to review and iterate on our policies and their enforcement."

Facebook, by contrast, managed to issue a timely response within almost 94 percent of cases, according to the Commission's report. Its ability to provide timely feedback was echoed by Shapira, who said of the 150 comments he had reported to Facebook, 80 percent were removed within one to three days.

Social networks face stern laws in Germany

Twitter's sluggishness to adequately tackle hate speech could have major financial repercussions. In June, Germany became the first country to introduce legal guidelines against online hate speech, after the Bundestag passed a controversial new law imposing fines on social media companies that fail to delete hateful content within 24 hours.

Read more: Opinion: Social media monitoring law a step in the right direction

Justifying the legislation, German Justice Minister Heiko Maas, said the new law wasn't designed to curb freedom of expression but was instead a prerequisite for it. "We do not accept the fact that companies in Germany do not adhere to the law," he said.

Critics of the law, however, worry that it could lead to social networks deleting a flurry of flagged posts, just to be on the safe side, without properly checking them.