Minor scuffles with police broke out as they surged into the main road outside the Downing Street residence of British Prime Minister David Cameron who will hold talks with Netanyahu on Thursday morning.
Already 107,000 people in Britain have signed an online petition for Netanyahu's arrest after last year's fighting between Israel and Hamas in Gaza.
"We're here because we feel that Netanyahu should pay for his war crimes," said 21-year-old student Marion Tehami. "We're here to protest and let him know that he's not welcome in our country."
Read: Mother of West Bank toddler killed in arson attack dies of wounds
A short distance away in a separate cordoned off area, about 50 pro-Israel demonstrators waved the Israeli flag.
Britain says visiting heads of state have immunity from legal process and thus cannot be arrested.
Earlier, speaking after meeting European Council President Donald Tusk and before flying to London for talks with David Cameron, Netanyahu said: "Europe should support Israel - not pressure Israel, not attack Israel, but support Israel, which is the only real shield that Europe and the Middle East have against extremism, which is surging," he told reporters.
Some British supermarkets already label Israeli settlement goods and the British Foreign Office has posted warnings on its website about the risks of doing business with Israeli companies that operate in the West Bank.
In a statement, the government said on Tuesday: "We recognise that the conflict in Gaza last year took a terrible toll.
Read: Israel releases all suspects linked to deadly firebombing of Palestinian home
"However the prime minister was clear on the UK's recognition of Israel's right to take proportionate action to defend itself, within the boundaries of international humanitarian law."
More than 500 children were among the 2,100 Palestinians, the majority of them civilians, who were killed in last year's conflict. Seventy-three Israelis, almost all soldiers, were killed.
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