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Alphabet's stock plunged almost 8% on Tuesday, slashing the market cap of Google's parent company by about $70 billion to $831 billion.

Alphabet reported a 17% rise in first-quarter revenue, its slowest sales growth in three years.

Earnings per share of $11.90 bested consensus forecasts, excluding a €1.5 billion fine ($1.7 billion) by European competition regulators.

Growth in Google's advertising sales and paid clicks slowed, and cost-per-click fell 19%.

George Salmon, an equity analyst at Hargreaves Lansdown, said it's "a nasty combination of growth in traffic to Google ads slowing and lower revenue per click from those ads that's upset the market."

Watch Alphabet trade live.

Shares in Alphabet plunged about 8% on Tuesday, slashing the market cap of Google's parent company by around $70 billion to $831 billion, as investors lashed out after a mixed first-quarter earnings report.

Alphabet — which also owns YouTube, Waymo and DeepMind — reported a 17% rise in revenue to $36.34 billion, its slowest sales growth in three years. However, earnings per share climbed to $11.90 versus consensus forecasts of $10.53, excluding a €1.5 billion fine ($1.7 billion) by European competition regulators.

"The market may be punishing Alphabet just a little harshly when you consider the impact of FX headwinds in these numbers," said Neil Wilson, chief market analyst for Markets.com.

Google's advertising revenue rose by 15% to $30.72 billion, a sharp slowdown from 24% growth a year ago, according to its earnings report for the first quarter of 2018. Paid clicks rose 39%, a significant decrease from 59% year-on-year growth in the first quarter of 2018. Cost-per-click also fell 19%, after sliding 19% in the same period of 2018.

"Another EU fine won't have washed well with investors, but in reality it's not the check on its way to Brussels that's causing the shares to drop," said George Salmon, an equity analyst at Hargreaves Lansdown.

"Instead, it's a nasty combination of growth in traffic to Google ads slowing and lower revenue per click from those ads that's upset the market."

Mounting losses at Google's famous "moonshots" won't have helped matters. Sales in Alphabet's "Other Bets" segment rose 13% to $170 million, but the division's operating loss widened by 52% to $868 million.

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